MySQL Administrator''''s Bible- P15

The MySQL Data
Dictionary
IN THIS CHAPTER
Examining the Object catalog
System information
Managing permissions
Developing custom metadata
M
ySQL stores information about the data in the databases; this
is called metadata. Much of this information is stored in the
INFORMATION_SCHEMA
database, following the SQL 2003
standard.
Tables in the
INFORMATION_SCHEMA
database are read-only, in-memory,
and show data from various sources.
SHOW CREATE TABLE
will show the tables as
TEMPORARY
, because
they reside in memory and do not persist between
mysqld
restarts.
INFORMATION_SCHEMA
tables are called system views and they may be of
different storage engine types. At the time of this writing all the system
views are either the MEMORY, MyISAM, or Maria storage engine.
Regular SQL statements can be used to query them, though they have
some special properties that other views do not have:
■

mysqldump
will not export any information (data, schema) from
INFORMATION_SCHEMA
system views
■ There is no data directory for the
INFORMATION_SCHEMA
database
■ There is no
.frm
file associated with the
INFORMATION_SCHEMA
views. The definitions are hard-coded into the database.
The table definitions for the data dictionary are hard-coded into the source
code, and loaded when
mysqld
starts. Unlike other databases, there is
no directory in the
datadir
for the
INFORMATION_SCHEMA
database.
All users have permission to see the
INFORMATION_SCHEMA
database;
however, they can only see the objects they have permission to see. For
example, table details in the
TABLES
system view are limited to the tables
that the user has permission to see.
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Some of the metadata provided by the
INFORMATION_SCHEMA
database is also provided by
various
SHOW
commands (see Chapter 4). The
INFORMATION_SCHEMA
database is a more
complete data dictionary than using
SHOW
commands. Also, standard SQL statements can be
used to query the system views to retrieve metadata.
The
INFORMATION_SCHEMA
database contains more than 40 system views. They can be infor-
mally categorized as:
■ Object catalog (databases, tables, columns, and so on)
■ System information (variables, statistics, available options)
■ Permissions
■ Storage engine-specific metadata
Querying metadata is a powerful tool for a database administrator, answering simple questions
such as ‘‘how many tables of each storage engine type exist?’’ and ‘‘which tables have columns
using the
DECIMAL
type?’’ and ‘‘how many foreign key constraints exist?’’ and even ‘‘how much
space does a certain group of three tables use?’’ Querying metadata provides a way to retrieve
information about the system that can be used to track and tune performance. Any tool that per-

forms queries can retrieve metadata by querying the
INFORMATION_SCHEMA
database, exactly
the same way it queries any other database.
Object Catalog
The
INFORMATION_SCHEMA
database contains system views with metadata about objects such as
databases, tables, views, columns, indexes, partitions, stored routines, triggers, and events.
SCHEMATA
‘‘Schema’’ is another name for a database, and ‘‘schemata’’ is the plural of schema. The
SCHEMATA
system view in the
INFORMATION_SCHEMA
database provides information about all
the databases, including the
mysql
system database and the
INFORMATION_SCHEMA
database
itself. The fields in the
SCHEMATA
system view are:
■
CATALOG_NAME
— Provided for standards compliance. However, because MySQL does
not have catalogs, this value is always
NULL
.
■

SCHEMA_NAME
— The name of the database, such as
sakila
.
■
DEFAULT_CHARACTER_SET_NAME
— The default character set of the database. If no
default character set is assigned by a
CREATE DATABASE
or
ALTER DATABASE
command,
the default character set for the system is stored. Thus, the
DEFAULT_CHARACTER_SET_
NAME
field always has a non-
NULL
value, and defaults to the character set of the system at
the time of database creation.
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21
■
DEFAULT_COLLATION_NAME
— The default collation of the database. If no default
collation is assigned by a
CREATE DATABASE
or
ALTER DATABASE

command, the default
collation for the system is stored. Thus, the
DEFAULT_CHARACTER_SET_NAME
field
always has a non-
NULL
value, and defaults to the collation of the system at the time of
database creation.
■
SQL_PATH
— Provided for standards compliance, this field is usually used to find files
related to the database. However, MySQL does not support this field, so it is always
NULL
.
The
SHOW DATABASES
command is a shorter way to find the names of existing databases than
running
SELECT SCHEMA_NAME FROM SCHEMATA
. To show a subset of all databases, it is easier
to use the
SCHEMATA
system view. The
SHOW DATABASES
command returns a result set where
the field name is
Database
. Because
Database
is a reserved word, in order to use the

WHERE
extension to
SHOW DATABASES
,the
Database
field must be quoted:
mysql> SHOW DATABASES WHERE Database NOT IN (’mysql’,’information_
schema’);
ERROR 1064 (42000): You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near ’NOT IN (’mysql’,’information_schema’)’ at line 1
mysql> SHOW DATABASES WHERE `Database` NOT IN (’mysql’,’information_
schema’);
+----------+
| Database |
+----------+
| sakila |
| test |
+----------+
2 rows in set (0.02 sec)
See Chapter 4 for more information about using the backtick (
`
) to quote identifiers.
The field name in the
SCHEMATA
system view is
SCHEMA_NAME
, which is not a reserved word,
and does not need to be escaped:
mysql> USE INFORMATION_SCHEMA;

Database changed
mysql> SELECT SCHEMA_NAME FROM SCHEMATA WHERE SCHEMA_NAME
NOT IN (’mysql’,’information_schema’);
+-------------+
| SCHEMA_NAME |
+-------------+
| sakila |
| test |
+-------------+
2 rows in set (0.02 sec)
The
SHOW DATABASES
command also accepts the
LIKE
extension. The
SHOW SCHEMAS
command behaves the same way as the
SHOW DATABASES
command, and outputs the same
information —
SCHEMAS
is an alias for
DATABASES
.
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The
SHOW CREATE DATABASE

command returns two fields:
Database
, which is equivalent to
SCHEMA_NAME
,and
Create Database
, which does not have an exact equivalent but contains
the value of
DEFAULT_CHARACTER_SET_NAME
.
SCHEMA
is an alias for
DATABASE
,so
SHOW
CREATE SCHEMA
returns the same information as
SHOW CREATE DATABASE
does.
TABLES
Metadata about non-temporary tables is available in the
TABLES
system view. The fields in the
TABLES
system view are:
■
TABLE_CATALOG
— Provided for standards compliance. However, because MySQL does
not have catalogs, this value is always
NULL

.
■
TABLE_SCHEMA
— The name of the database, such as
sakila
.
■
TABLE_NAME
— The name of the table.
■
TABLE_TYPE
— Whether the table is a base table, view, or system view. Only the
INFOR-
MATION_SCHEMA
views are system views.
■
ENGINE
— The storage engine of the table, such as InnoDB. To find out which tables, if
any, in the
INFORMATION_SCHEMA
database do not use the MEMORY storage engine:
mysql> SELECT TABLE_NAME,ENGINE FROM TABLES WHERE TABLE_
SCHEMA=’INFORMATION_SCHEMA’ AND ENGINE!=’MEMORY’;
+-------------+--------+
| TABLE_NAME | ENGINE |
+-------------+--------+
| COLUMNS | MARIA |
| EVENTS | MARIA |
| PARAMETERS | MARIA |
| PARTITIONS | MARIA |

| PLUGINS | MARIA |
| PROCESSLIST | MARIA |
| ROUTINES | MARIA |
| TRIGGERS | MARIA |
| VIEWS | MARIA |
+-------------+--------+
9 rows in set (0.56 sec)
■ From this query we see that most system views in the
INFORMATION_SCHEMA
database
are the MEMORY storage engine, but there are some that use the Maria storage engine.
■
VERSION
— The version of the
.frm
file, currently 10. The
VERSION
is
NULL
for table
objects that do not have
.frm
files, such as views. The exception to this rule is system
views, which have a
VERSION
of 10 — even though there are no
.frm
files, system views
have hard-coded definitions, and thus have versions.
■

ROW_FORMAT
— Different storage engines allow the row storage to vary. Fixed-width
rows are a fixed size, which minimizes fragmentation. Dynamic rows are a variable size,
which are good for variable-length data, such as
VARCHAR
,
TEXT
,and
BLOB
. InnoDB has
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The MySQL Data Dictionary
21
a compact row format by default, which eliminates some redundant data. When an Inn-
oDB table has a redundant format, there is less CPU work needed at the cost of additional
storage space. MyISAM has a compressed format, obtained by packing the data with the
myisampack
tool. See Chapter 11 for more details on
myisampack
.
■
TABLE_ROWS
— The number of rows this table contains. This value may be an estimate,
depending on the storage engine. The value is
NULL
for views and system views
(
INFORMATION_SCHEMA
tables).

■
AVG_ROW_LENGTH
— The average size in bytes of rows this table contains. This value may
be an estimate, depending on the storage engine. The value is
NULL
for views, but has a
value for system views. If there are no rows, the value will be 0.
■
DATA_LENGTH
— The size in bytes of rows this table contains. This value may be an esti-
mate, depending on the storage engine. The value is
NULL
for views and 0 for MEMORY
tables. System views that are not MEMORY tables have a value for
DATA_LENGTH
.
■
MAX_DATA_LENGTH
— The maximum size in bytes that this table may contain. The value
is
NULL
for views, because there is no data stored. The value is 0 for storage engines that
do not populate this field, such as Falcon and CSV.
■
INDEX_LENGTH
— The size in bytes of the indexes for this table. This value may be an
estimate, depending on the storage engine. The value is
NULL
for views and 0 for MEM-
ORY tables. System views that are not MEMORY tables have a value for

DATA_LENGTH
.
■
DATA_FREE
— The size in bytes of the free space allocated for this table, and still avail-
able. This value may be an estimate, depending on the storage engine. The value is
NULL
for views and 0 for system views. Many tables have a
DATA_FREE
value of 0 because there
is not space allocated for them, though there may be plenty of free space available to
them. For example, CSV tables simply use available disk space, without needing MySQL
to allocate space for rows. In some storage engines such as MyISAM, this might indicate
fragmentation and that the table needs to be rebuilt with an
OPTIMIZE
command. See
Chapter 4 for more information about
OPTIMIZE
.
■
AUTO_INCREMENT
— The next
AUTO_INCREMENT
valuetobeused.Ifthemaximum
AUTO_INCREMENT
value for a table is 100, the value of
AUTO_INCREMENT
is 101. If a
table has an
AUTO_INCREMENT

value and no rows have ever been stored in the table, the
value of
AUTO_INCREMENT
is 1.
■
CREATE_TIME
—The
DATETIME
the table was created. The value is
NULL
for views and
MEMORY tables. System views that use storage engines other than MEMORY have a
proper
DATETIME
value. FEDERATED tables have a value of
NULL
.
■
UPDATE_TIME
— The most recent
DATETIME
that an
ALTER TABLE
was performed on
the table. The value is
NULL
for views, CSV, and MEMORY tables. MyISAM, Archive, and
Maria tables that have never had
ALTER TABLE
performed on them have an

UPDATE_TIME
equivalent to their
CREATE_TIME
. InnoDB and Falcon tables that have never had
ALTER
TABLE
performed on them have a
NULL
value. System views that use storage engines
other than MEMORY have a proper
DATETIME
value. FEDERATED tables have a value
of
NULL
.
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■
CHECK_TIME
— The most recent
DATETIME
the table was checked with
CHECK TABLE
.
The value is
NULL
for views, system views, and tables that have never been checked or do
not support the check function.

■
TABLE_COLLATION
— The character set and collation of the table, for example
utf8_bin utf8_general_ci
,or
latin1_swedish_ci
.Thevalueis
NULL
for views.
If no default character set and collation is assigned by a
CREATE TABLE
or
ALTER TABLE
command, the default character set and collation are stored. Thus, this field always has a
non-
NULL
value for base tables and system views.
■
CHECKSUM
— Live checksums can be maintained for MyISAM tables (see Chapter 11). If
this table is a MyISAM table with
CHECKSUM=1
, the live checksum value is displayed. For
all other tables, the value is
NULL
.
■
CREATE_OPTIONS
—
CREATE TABLE

has many different options. The options that are not
shown in other fields (such as
TABLE_COLLATION
) are shown in this field, separated by a
space. Sample values are
partitioned
and
max_rows=10000 checksum=1
.Ifthereare
no relevant options to
CREATE TABLE
, the value is the empty string (’’). The value is
NULL
for views.
■
TABLE_COMMENT
—The
COMMENT
option to
CREATE TABLE
and
ALTER TABLE
can be
used to provide information about a table. If there was no comment specified, the value is
the empty string (‘’). The value is
VIEW
for views. Most of the tables in the
mysql
system
database have comments:

mysql> SELECT TABLE_NAME, TABLE_COMMENT FROM TABLES WHERE
TABLE_SCHEMA=’mysql’\G
*************************** 1. row ***************************
TABLE_NAME: backup_history
TABLE_COMMENT:
*************************** 2. row ***************************
TABLE_NAME: backup_progress
TABLE_COMMENT:
*************************** 3. row ***************************
TABLE_NAME: columns_priv
TABLE_COMMENT: Column privileges
*************************** 4. row ***************************
TABLE_NAME: db
TABLE_COMMENT: Database privileges
*************************** 5. row ***************************
TABLE_NAME: event
TABLE_COMMENT: Events
*************************** 6. row ***************************
TABLE_NAME: func
TABLE_COMMENT: User defined functions
*************************** 7. row ***************************
TABLE_NAME: general_log
TABLE_COMMENT: General log
*************************** 8. row ***************************
TABLE_NAME: help_category
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TABLE_COMMENT: help categories

*************************** 9. row ***************************
TABLE_NAME: help_keyword
TABLE_COMMENT: help keywords
*************************** 10. row ***************************
TABLE_NAME: help_relation
TABLE_COMMENT: keyword-topic relation
*************************** 11. row ***************************
TABLE_NAME: help_topic
TABLE_COMMENT: help topics
*************************** 12. row ***************************
TABLE_NAME: host
TABLE_COMMENT: Host privileges; Merged with database privileges
*************************** 13. row ***************************
TABLE_NAME: ndb_binlog_index
TABLE_COMMENT:
*************************** 14. row ***************************
TABLE_NAME: plugin
TABLE_COMMENT: MySQL plugins
*************************** 15. row ***************************
TABLE_NAME: proc
TABLE_COMMENT: Stored Procedures
*************************** 16. row ***************************
TABLE_NAME: procs_priv
TABLE_COMMENT: Procedure privileges
*************************** 17. row ***************************
TABLE_NAME: servers
TABLE_COMMENT: MySQL Foreign Servers table
*************************** 18. row ***************************
TABLE_NAME: slow_log
TABLE_COMMENT: Slow log

*************************** 19. row ***************************
TABLE_NAME: tables_priv
TABLE_COMMENT: Table privileges
*************************** 20. row ***************************
TABLE_NAME: time_zone
TABLE_COMMENT: Time zones
*************************** 21. row ***************************
TABLE_NAME: time_zone_leap_second
TABLE_COMMENT: Leap seconds information for time zones
*************************** 22. row ***************************
TABLE_NAME: time_zone_name
TABLE_COMMENT: Time zone names
*************************** 23. row ***************************
TABLE_NAME: time_zone_transition
TABLE_COMMENT: Time zone transitions
*************************** 24. row ***************************
TABLE_NAME: time_zone_transition_type
TABLE_COMMENT: Time zone transition types
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*************************** 25. row ***************************
TABLE_NAME: user
TABLE_COMMENT: Users and global privileges
25 rows in set (0.00 sec)
There are a few
SHOW
commands that show table information.
SHOW TABLES

returns one field,
the equivalent of
TABLE_NAME
.
SHOW FULL TABLES
adds another field,
Table_type
,which
is the equivalent of
TABLE_TYPE
.
The
SHOW CREATE TABLE
command returns two fields:
Table
, which is the equivalent of
TABLE_NAME
,and
Create Table
, which is the full
CREATE TABLE
statement. There is
no equivalent in the
TABLES
system view, though it includes the information in
ENGINE
,
AUTO_INCREMENT
,
CREATE_OPTIONS

,and
TABLE_COMMENT
.
SHOW TABLE STATUS
returns many fields:
■
Name
— Equivalent to
TABLE_NAME
.
■
Engine
,
Version
,and
Row_format
— Equivalent to
ENGINE
,
VERSION
,and
ROW_FORMAT
.
■
Rows
— Equivalent to
TABLE_ROWS
.
■
Avg_row_length

,
Data_length
,
Max_data_length
,
Index_length
,
Data_free
,
Auto_increment
,
Create_time
,
Update_time
,and
Check_time
—Equivalentto
AVG_ROW_LENGTH
,
DATA_LENGTH
,
MAX_DATA_LENGTH
,
INDEX_LENGTH
,
DATA_FREE
,
AUTO_INCREMENT
,
CREATE_TIME

,
UPDATE_TIME
,and
CHECK_TIME
.
■
Collation
— Equivalent to
TABLE_COLLATION
.
■
Checksum
,
Create_options
— Equivalent to
CHECKSUM
and
CREATE_OPTIONS
.
■
Comment
— Equivalent to
TABLE_COMMENT
.
VIEWS
The
TABLES
system view includes rows for views. However, many of the fields in
TABLES
are

NULL
for views, and some features specific to views are not encompassed in the
TABLES
system
view. So MySQL provides the
VIEWS
system view, with the following fields:
■
TABLE_CATALOG
— Provided for standards compliance. However, because MySQL does
not have catalogs, this value is always
NULL
.
■
TABLE_SCHEMA
— The name of the database, such as
sakila
.
■
TABLE_NAME
— The name of the view.
■
VIEW_DEFINITION
—The
SELECT
statement that defines the view. If the current user is
not the definer, the value will be blank, even if the current user has permissions to see the
view definition:
mysql> SELECT DEFINER,VIEW_DEFINITION,CURRENT_USER()
-> FROM VIEWS

-> WHERE TABLE_NAME=’staff_list’;
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+----------------+-----------------+----------------+
| DEFINER | VIEW_DEFINITION | CURRENT_USER() |
+----------------+-----------------+----------------+
| root@localhost | | |
+----------------+-----------------+----------------+
1 row in set (0.03 sec)
mysql> SHOW GRANTS;
+-------------------------------------------------------------+
| Grants for |
+-------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO ’root’@’127.0.0.1’ WITH GRANT|
| OPTION |
+-------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> SHOW CREATE VIEW sakila.staff_list\G
*************************** 1. row ***************************
View: staff_list
Create View: CREATE ALGORITHM=UNDEFINED DEFINER=root@
localhost SQL SECURITY DEFINER VIEW sakila.staff_list AS select
s.staff_id AS ID,concat(s.first_name,_utf8’ ’,s.last_name) AS name,
a.address AS address,a.postal_code AS zip code,a.phone AS phone,
sakila.city.city AS city,sakila.country.country AS country,s.
store_id AS SID from (((sakila.staff s join sakila.address a
on((s.address_id = a.address_id))) join sakila.city on((a.city_id
= sakila.city.city_id))) join sakila.country on((sakila.city.

country_id = sakila.country.country_id)))
character_set_client: utf8mb3
collation_connection: utf8mb3_general_ci
1 row in set (0.00 sec)
■ In this example, the user

saw a blank view definition for the
staff_list
view, because the
DEFINER
is
root@localhost
.
■
CHECK_OPTION
— This value is
NONE
if the view definition has no
WITH CHECK OPTION
clause;
CASCADED
if the view definition contains
WITH [CASCADED] CHECK OPTION
,and
LOCAL
if the view definition contains
WITH LOCAL CHECK OPTION
.
■
IS_UPDATABLE

—
YES
if the view is updatable,
NO
if the view is not updatable. See
Chapter 8, subsection ‘‘Updatable Views,’’ for more information on updatable views.
■
DEFINER
— The view definer, in the MySQL
user@host
format.
■
SECURITY_TYPE
—
DEFINER
if the view definition was specified with the
SQL
SECURITY DEFINER
option or did not contain an
SQL SECURITY
option. The value is
INVOKER
if the view definition was specified with the
SQL SECURITY INVOKER
option.
■
CHARACTER_SET_CLIENT
— Stores the environmental character set as it was when the
view was created.
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■
COLLATION_CONNECTION
— Stores the environmental collation as it was when the view
was created.
SHOW CREATE VIEW
is the
SHOW
command that shares the most information with the
VIEWS
sys-
tem view. The fields of
SHOW CREATE VIEW
are:
■
View
— Equivalent to
TABLE_NAME
.
■
Create View
— No exact equivalent. This is the full
CREATE VIEW
statement, and has
elements from
TABLE_NAME
,
VIEW_DEFINITION

,
CHECK_OPTION
,
IS_UPDATABLE
,
DEFINER
,and
SECURITY_TYPE
.
■
character_set_client
— Equivalent to
CHARACTER_SET_CLIENT
.
■
collation_connection
— Equivalent to
COLLATION_CONNECTION
.
COLUMNS
The
COLUMNS
system view contains information about table fields. This system view contains
information about the fields from every table, view, and system view.
■
TABLE_CATALOG
— Provided for standards compliance. However, because MySQL does
not have catalogs, this value is always
NULL
.

■
TABLE_SCHEMA
— The name of the database, such as
sakila
.
■
TABLE_NAME
— The name of the table.
■
COLUMN_NAME
— The name of the field.
■
ORDINAL_POSITION
— The number representing the order of the field. The first field has
a value of 1, the third field has a value of 3, and so on. The value is never
NULL
.
■
COLUMN_DEFAULT
— The default value of the field. If the default is not specified or speci-
fied as
NULL
,thevalueis
NULL
.
■
IS_NULLABLE
— Whether or not the field is allowed to be null. If the field is specified as
NOT NULL
,thevalueis

NO
.Otherwise,thevalueis
YES
. Note that it is possible to have a
table where the value of
IS_NULLABLE
is
NO
and the
COLUMN_DEFAULT
is
NULL
:
mysql> USE test;
Database changed
mysql> CREATE TABLE paradox (numfield INT NOT NULL);
Query OK, 0 rows affected (0.11 sec)
mysql> SELECT IS_NULLABLE,COLUMN_DEFAULT
-> FROM INFORMATION_SCHEMA.COLUMNS
-> WHERE TABLE_NAME=’paradox’ AND TABLE_SCHEMA=’test’;
+-------------+----------------+
| IS_NULLABLE | COLUMN_DEFAULT |
+-------------+----------------+
| NO | NULL |
+-------------+----------------+
1 row in set (0.00 sec)
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21

■
DATA_TYPE
— The data type of the field, such as
INT
,
CHAR
,or
ENUM
.
■
CHARACTER_MAXIMUM_LENGTH
— The maximum number of characters allowed by
the field definition. For example, a field defined as
VARCHAR(64)
has a
CHARAC-
TER_MAXIMUM_LENGTH
of 64. This is only valid for string data, which includes
CHAR
,
VARCHAR
,
TEXT
,
BLOB
,
SET
,and
ENUM
types.

■
CHARACTER_OCTET_LENGTH
— Maximum octet (byte) size allowed by the field defini-
tion.
CHARACTER_OCTET_LENGTH
is the same as
CHARACTER_MAXIMUM_LENGTH
except
when multi-byte character sets are used (such as
utf8
).
■
NUMERIC_PRECISION
— Mathematically, precision is the number of digits used to define
a number, for example 10 for an
INT
and 5 for a
SMALLINT
. This is only valid for numeric
data types, including
INT
,
DECIMAL
,and
FLOAT
. For all other data types, the value is
NULL
.
■
NUMERIC_SCALE

— This is only valid for numeric data types, including
INT
,
DECIMAL
,
and
FLOAT
. For all other data types, the value is
NULL
. This value is equivalent to the
number of digits after the decimal point, which is 0 for data types that are integers.
■
CHARACTER_SET_NAME
— The default character set for this column. This is only valid for
non-binary string data, which includes
CHAR
,
VARCHAR
,
TEXT
,
SET
,and
ENUM
types.
■
COLLATION_NAME
— The default collation for this column. This is only valid for
non-binary string data, which includes
CHAR

,
VARCHAR
,
TEXT
,
SET
,and
ENUM
types.
Sample values are
utf8_general_ci
,
latin1_bin
,and
latin1_swedish_ci
.
■
COLUMN_TYPE
— The entire data type part of the column definition, without any
NULL
or
DEFAULT
definers. For example,
varchar(64)
,
bigint(21) unsigned
,
enum(’N’,’Y’)
,or
year(4)

.
■
COLUMN_KEY
— If the column is not part of an index, this field contains the empty string
(‘’). Otherwise, the value is the type of index the columns is a part of:
■
PRI
— Primary key
■
UNI
— Unique, non-primary key
■
MUL
— Non-unique key
■
EXTRA
— This field stores extra information about the column that does not have a place
in another field. If there is no such extra information, the value is the empty string (‘’).
Example values are
on update CURRENT_TIMESTAMP
and
auto_increment
.
■
PRIVILEGES
— The privileges the querying user has for use with this column. If the user
has no privileges for a column, there is no row returned for that column. Therefore, the
field always contains at least the
select
privilege. Privileges are separated by a comma,

for example
select,insert,update
.
■
COLUMN_COMMENT
—The
COMMENT
option to a column definition within
CREATE TABLE
and
ALTER TABLE
can be used to provide information about a column. If there was no
comment specified, the value is the empty string (‘’).
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■
STORAGE
— Indicates whether the column is stored in memory or on disk. These only
apply to columns in NDB tables. The value for all other tables is
Default
.
■
FORMAT
— Indicates whether the column storage format is fixed, dynamic, or default.
Fixed and dynamic storage formats only apply to columns in NDB tables. The value for all
other tables is
Default
.

The
SHOW COLUMNS
command is the
SHOW
command that shares the most information with the
COLUMNS
system view. It accepts the
LIKE
and
WHERE
extensions, and will show three extra
fields when the
SHOW FULL COLUMNS
syntax is used:
mysql> SHOW COLUMNS FROM sakila.staff LIKE ’s%’\G
*************************** 1. row ***************************
Field: staff_id
Type: tinyint(3) unsigned
Null: NO
Key: PRI
Default: NULL
Extra: auto_increment
*************************** 2. row ***************************
Field: store_id
Type: tinyint(3) unsigned
Null: NO
Key: MUL
Default: NULL
Extra:
2 rows in set (0.00 sec)

mysql> SHOW FULL COLUMNS FROM sakila.staff LIKE ’s%’\G
*************************** 1. row ***************************
Field: staff_id
Type: tinyint(3) unsigned
Collation: NULL
Null: NO
Key: PRI
Default: NULL
Extra: auto_increment
Privileges: select,insert,update,references
Comment:
*************************** 2. row ***************************
Field: store_id
Type: tinyint(3) unsigned
Collation: NULL
Null: NO
Key: MUL
Default: NULL
Extra:
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Privileges: select,insert,update,references
Comment:
2 rows in set (0.00 sec)
The fields from
SHOW COLUMNS
and
SHOW FULL COLUMNS

are:
■
Field
— Equivalent to
COLUMN_NAME
.
■
Type
— Equivalent to
COLUMN_TYPE
.
■
Collation
— Equivalent to
COLLATION_NAME
.
SHOW FULL COLUMNS
only.
■
Null
— Equivalent to
IS_NULLABLE
.
■
Key
— Equivalent to
COLUMN_KEY
.
■
Default

— Equivalent to
COLUMN_DEFAULT
.
■
Extra
— Equivalent to
EXTRA
.
■
Privileges
— Equivalent to
PRIVILEGES
.
SHOW FULL COLUMNS
only.
■
Comment
— Equivalent to
COLUMN_COMMENT
.
SHOW FULL COLUMNS
only.
SHOW COLUMNS
returns the fields in the order in which they appear in the table. To guarantee
that ordering with the
COLUMNS
system view, add
ORDER BY ORDINAL_POSITION
to queries.
STATISTICS

Information about indexes is stored in the
STATISTICS
system view, which has one row for
each field in an index.
STATISTICS
has the following fields:
■
TABLE_CATALOG
— Provided for standards compliance. However, because MySQL does
not have catalogs, this value is always
NULL
.
■
TABLE_SCHEMA
— The name of the database that contains the table that is associated with
this index, such as
sakila
.
■
TABLE_NAME
— The name of the table that is associated with this index.
■
NON_UNIQUE
— Whether or not the index is unique. The value is 0 for unique indexes
and 1 for non-unique indexes.
■
INDEX_SCHEMA
— The name of the database that contains this index. This is always the
same as
TABLE_SCHEMA

.
■
INDEX_NAME
— The name of the index.
■
SEQ_IN_INDEX
— The position of this field in the index. The first field in an index has
a value of 1, the second field in an index has a value of 2, and so on. For example, the
sakila.rental
table has an index defined as:
UNIQUE KEY `rental_date` (`rental_date`,`inventory_id`,
`customer_id`),
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This index is represented by three rows in the
STATISTICS
system view:
mysql> SELECT SEQ_IN_INDEX, COLUMN_NAME
-> FROM STATISTICS
-> WHERE TABLE_SCHEMA=’sakila’
-> AND TABLE_NAME=’rental’
-> AND INDEX_NAME=’rental_date’;
+--------------+--------------+
| SEQ_IN_INDEX | COLUMN_NAME |
+--------------+--------------+
| 1 | rental_date |
| 2 | inventory_id |
| 3 | customer_id |

+--------------+--------------+
3 rows in set (0.00 sec)
■
COLUMN_NAME
— The name of the field.
■
COLLATION
— The collation for this field. Currently all records have
NULL
values. How-
ever, in the future, when
mysqld
supports ascending and descending collations, the value
will be
A
for ascending collations and
D
for descending collations.
■
CARDINALITY
— The cardinality of this field. The cardinality of a field is the number of
unique values in that field. This value may be an estimation, depending on the storage
engine of the table. If the cardinality is small compared to the number of rows in the table,
it means that there are many repeating values. This means a query will return multiple
values for a field when searching on a value that is indexed, but repeating.
When the field cardinality is large, approaching the value of the number of rows, it means
that there are very few repeating values. Fields in a unique index have a cardinality equal
to the number of rows in the table.
■
SUB_PART

— The number of characters in the prefix of the index for this field. If
the index does not contain a prefix for this field, the value is
NULL
. For example, the
sakila.film
table has the following index defined:
KEY `idx_title` (`title`(191)),
And the value of
SUB_PART
is 191:
mysql> SELECT INDEX_NAME, COLUMN_NAME, SUB_PART
-> FROM STATISTICS
-> WHERE TABLE_SCHEMA=’sakila’
-> AND TABLE_NAME=’film’
-> AND INDEX_NAME=’idx_title’;
+------------+-------------+----------+
| INDEX_NAME | COLUMN_NAME | SUB_PART |
+------------+-------------+----------+
| idx_title | title | 191 |
+------------+-------------+----------+
1 row in set (0.00 sec)
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■
PACKED
— Whether or not the index is packed. This only applies to MyISAM tables. If
the index is not packed, the value is
NULL

. If the index is packed, the value is
0
(nothing
packed),
1
(strings and integers are packed), or
DEFAULT
(only strings are packed).
■
NULLABLE
— Whether or not the field can contain
NULL
values. If the field can contain
NULL
values, the value is
YES
. If the field cannot contain
NULL
values, the value is the
empty string (‘’).
■
INDEX_TYPE
— The type of index. Example values are
BTREE
,
FULLTEXT
,
HASH
,and
RTREE

.
■
COMMENT
— Always the empty string (‘’).
■
INDEX_COMMENT
— The comment defined by the
index_comment
option when creating
or changing an index.
The output of
SHOW INDEX
is very similar to the fields in the
STATISTICS
system view:
■
Table
— Equivalent to
TABLE_NAME
.
■
Non_unique
— Equivalent to
NON_UNIQUE
.
■
Key_name
— Equivalent to
INDEX_NAME
.

■
Seq_in_index
,
Column_name
,
Collation
,
Cardinality
,
Sub_part
,
Packed
,
Null
,
Index_type
,
Comment
,and
Index_Comment
— Equivalent to
SEQ_IN_INDEX
,
COLUMN_NAME
,
COLLATION
,
CARDINALITY
,
SUB_PART

,
PACKED
,
NULLABLE
,
INDEX_TYPE
,
COMMENT
,and
INDEX_COMMENT
.
TABLE_CONSTRAINTS
Unique and primary keys restrict data values in a table, only allowing one set of values for the
fields in those indexes. A foreign key restricts the allowable data for fields in a table by only
allowing values from another set of fields. The restrictions that unique, primary, and foreign
keys place on tables are referred to as table constraints. The
TABLE_CONSTRAINTS
system view
has information about unique keys, primary keys, and foreign keys in the following fields:
■
CONSTRAINT_CATALOG
— Provided for standards compliance. However, because MySQL
does not have catalogs, this value is always
NULL
.
■
CONSTRAINT_SCHEMA
— The name of the database that the constraint belongs to, such
as
sakila

.Thisisthesameasthe
TABLE_SCHEMA
field.
■
CONSTRAINT_NAME
— The name of the index. The value is
PRIMARY
for a primary key.
Unique keys and foreign keys have names that can be set when an index is created or
changed. The default name for foreign keys starts with
fk_
.
■
TABLE_SCHEMA
— The name of the database that the constrained table belongs to.
■
TABLE_NAME
— The name of the table constrained.
■
CONSTRAINT_TYPE
— The type of constraint. Either
PRIMARY KEY
,
UNIQUE
,or
FOREIGN KEY
.
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There is no equivalent
SHOW
statement for the data in
TABLE_CONSTRAINTS
, but the output of
SHOW INDEX
has some of the same fields:
■
Table
— Equivalent to
TABLE_NAME
.
■
Key_name
— Equivalent to
CONSTRAINT_NAME
for unique and primary keys. For foreign
keys, the
Key_name
is the name of the index on the field, and the
CONSTRAINT_NAME
is
the name of the foreign key constraint.
As an example, in the
sakila.staff
table, the
address_id
field is a foreign key, defined
with:

KEY idx_fk_address_id (address_id),
CONSTRAINT fk_staff_address FOREIGN KEY (address_id) REFERENCES
address (address_id) ON UPDATE CASCADE,
The
Key_name
from
SHOW INDEX
is
idx_fk_address_id
,andthe
CONSTRAINT_NAME
that
appears in the
TABLE_CONSTRAINTS
system view is
fk_staff_address
.
■
Column_name
— Equivalent to
COLUMN_NAME
.
Note that
SHOW INDEX
shows all indexes and
TABLE_CONSTRAINTS
has information only for
unique, primary, and foreign keys.
KEY_COLUMN_USAGE
Like the

TABLE_CONSTRAINTS
system view, the
KEY_COLUMN_USAGE
system view only
shows information about unique keys, primary keys, and foreign keys. The fields in
KEY_COLUMN_USAGE
are:
■
CONSTRAINT_CATALOG
— Provided for standards compliance. However, because MySQL
does not have catalogs, this value is always
NULL
.
■
CONSTRAINT_SCHEMA
— The name of the database that the constraint belongs to, such
as
sakila
.Thisisthesameasthe
TABLE_SCHEMA
field.
■
CONSTRAINT_NAME
— The name of the index. The value is
PRIMARY
for a primary key.
Unique keys and foreign keys have names that can be set when an index is created or
changed. The default name for foreign keys starts with
fk_
.

■
TABLE_CATALOG
— Provided for standards compliance. However, because MySQL does
not have catalogs, this value is always
NULL
.
■
TABLE_SCHEMA
— The name of the database that the constrained table belongs to.
■
TABLE_NAME
— The name of the table constrained.
■
COLUMN_NAME
— The name of the field constrained.
■
ORDINAL_POSITION
— The constrained field’s position in the index. The first field in
an index has a value of 1, the second field has a value of 2, and so on. For example, the
sakila.rental
table has a composite unique index (a unique index with more than one
field):
UNIQUE KEY rental_date (rental_date, inventory_id, customer_id)
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The
ORDINAL_POSITION
of

rental_date
is 1, of
inventory_id
is 2, and of
customer_id
is 3.
■
POSITION_IN_UNIQUE_CONSTRAINT
—
NULL
for keys that are not foreign keys. For
foreign keys, the value is the referenced field’s position in the foreign key.
As an example, the foreign key
fk_staff_address
on the
sakila.staff
table is defined as:
CONSTRAINT fk_staff_address FOREIGN KEY (address_id) REFERENCES
address (address_id) ON UPDATE CASCADE
The value of
POSITION_IN_UNIQUE_CONSTRAINT
is 1, referring to the fact that the
address_id
field is the first referenced field. Foreign keys are usually defined as:
FOREIGN KEY (fld1,fld2) REFERENCES reftbl (reffld1,reffld2)
Thus, the
POSITION_IN_UNIQUE_CONSTRAINT
is the same as
ORDINAL_POSITION
.

■
REFERENCED_TABLE_SCHEMA
—
NULL
for keys that are not foreign keys. For foreign
keys, the database name of the referenced table. As an example, the foreign key
fk_staff_address
on the
sakila.staff
table is defined as:
CONSTRAINT fk_staff_address FOREIGN KEY (address_id) REFERENCES
address (address_id) ON UPDATE CASCADE
The
REFERENCED_TABLE_SCHEMA
is
sakila
, which is the database containing the
address
table.
■
REFERENCED_TABLE_NAME
—
NULL
for keys that are not foreign keys. For foreign keys,
the name of the referenced table. In the previous example, the value is
address
.
■
REFERENCED_COLUMN_NAME
—

NULL
for keys that are not foreign keys. For foreign keys,
the name of the referenced field. In the previous example, the value is
address_id
.
There is no equivalent
SHOW
statement for the data in
KEY_COLUMN_USAGE
, but the output of
SHOW INDEX
has some of the same fields:
■
Table
— Equivalent to
TABLE_NAME
.
■
Key_name
— Equivalent to
CONSTRAINT_NAME
for unique and primary keys. For foreign
keys, the
Key_name
is the name of the index on the field, and the
CONSTRAINT_NAME
is
the name of the foreign key constraint.
As an example, in the
sakila.staff

table, the
address_id
field is a foreign key, defined
with:
KEY idx_fk_address_id (address_id),
CONSTRAINT fk_staff_address FOREIGN KEY (address_id) REFERENCES
address (address_id) ON UPDATE CASCADE,
The
Key_name
from
SHOW INDEX
is
idx_fk_address_id
,andthe
CONSTRAINT_NAME
that
appears in the
TABLE_CONSTRAINTS
system view is
fk_staff_address
.
■
Column_name
— Equivalent to
COLUMN_NAME
.
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Note that
SHOW INDEX
shows all indexes and
KEY_COLUMN_USAGE
has information only for
unique, primary, and foreign keys.
REFERENTIAL_CONSTRAINTS
There is more information about foreign keys than the
KEY_COLUMN_USAGE
and
TABLE_
CONSTRAINTS
system views show. The
REFERENTIAL_CONSTRAINTS
system view shows the
behavior of a foreign key during updates and deletes. These behaviors are defined in the foreign
key constraint with the
ON UPDATE
and
ON DELETE
clauses. The
REFERENTIAL_CONSTRAINTS
system view also repeats the constraint and referenced constraint:
■
CONSTRAINT_CATALOG
— Provided for standards compliance. However, because MySQL
does not have catalogs, this value is always
NULL
.
■

CONSTRAINT_SCHEMA
— The name of the database that the constraint belongs to, such
as
sakila
.
■
CONSTRAINT_NAME
— The name of the index. The value is
PRIMARY
for a primary key.
Unique keys and foreign keys have names that can be set when an index is created or
changed. The default name for foreign keys starts with
fk_
.
■
UNIQUE_CONSTRAINT_CATALOG
— Provided for standards compliance. However,
because MySQL does not have catalogs, this value is always
NULL
.
■
UNIQUE_CONSTRAINT_SCHEMA
— The database name of the referenced table. This is
equivalent to the
REFERENCED_TABLE_SCHEMA
in the
KEY_COLUMN_USAGE
system view.
■
UNIQUE_CONSTRAINT_NAME

— The name of the referenced constraint. Similar to the
CONSTRAINT_NAME
field of the
KEY_COLUMN_USAGE
system view, this value is
PRIMARY
for a primary key or the name of the index for non-primary keys.
■
MATCH_OPTION
— This feature has not been implemented yet, so the value is always
NONE
.
■
UPDATE_RULE
— The update behavior of the foreign key, as set by the foreign key defini-
tion. Possible values are
CASCADE
,
NO ACTION
,
RESTRICT
,
SET NULL
,
SET DEFAULT
.See
Chapter 6 for more information on defining foreign keys.
■
DELETE_RULE
— The delete behavior of the foreign key, as set by the foreign key defini-

tion. Possible values are
CASCADE
,
NO ACTION
,
RESTRICT
,
SET NULL
,
SET DEFAULT
.See
Chapter 6 for more information on defining foreign keys.
■
TABLE_NAME
— The table name that the foreign key constraint belongs to.
■
REFERENCED_TABLE_NAME
— The table name of the referenced table.
There is no complete
SHOW
statement for the fields in
REFERENTIAL_CONSTRAINTS
,butthe
Key_name
field from the output of
SHOW INDEX
is equivalent to
CONSTRAINT_NAME
.
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TRIGGERS
Triggers are user-defined stored SQL that get run when data changes. Triggers are defined on
a per-table basis, and can occur before and after inserts, updates, and deletes. The fields in the
TRIGGERS
system view are:
■
TRIGGER_CATALOG
— Provided for standards compliance. However, because MySQL
does not have catalogs, this value is always
NULL
.
■
TRIGGER_SCHEMA
— The name of the database that the trigger belongs to, such as
sakila
.
■
TRIGGER_NAME
— The name of the trigger.
■
EVENT_MANIPULATION
— The name of the action that calls this trigger. Possible values
are
INSERT
,
DELETE
,or

UPDATE
.
■
EVENT_OBJECT_CATALOG
— Provided for standards compliance. However, because
MySQL does not have catalogs, this value is always
NULL
.
■
EVENT_OBJECT_SCHEMA
— The name of the database that the table associated with the
trigger belongs to, such as
sakila
.
■
EVENT_OBJECT_TABLE
— The name of the table associated with the trigger.
■
ACTION_ORDER
— The order the trigger is run in. This value is always 0 because MySQL
only supports one trigger per set of
ACTION_TIMING
and
EVENT_MANIPULATION
.
■
ACTION_CONDITION
—Always
NULL
.

■
ACTION_STATEMENT
— The SQL that is run by the trigger.
■
ACTION_ORIENTATION
—Always
ROW
.
■
ACTION_TIMING
— When the trigger occurs. Possible values are
BEFORE
or
AFTER
.
■
ACTION_REFERENCE_OLD_TABLE
—Always
NULL
.
■
ACTION_REFERENCE_NEW_TABLE
—Always
NULL
.
■
ACTION_REFERENCE_OLD_ROW
— How the trigger references the row prior to the data
change. This is not settable by the user, and is always
OLD

.
■
ACTION_REFERENCE_NEW_ROW
— How the trigger references the row after the data is
changed. This is not settable by the user, and is always
NEW
.
■
CREATED
—Always
NULL
.
■
SQL_MODE
— Stores the environmental
sql_mode
as it was when the trigger was created.
This is also the
sql_mode
that is used when the trigger is invoked.
■
DEFINER
— The trigger definer, in the MySQL
user@host
format.
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CHARACTER_SET_CLIENT
— Stores the environmental character set as it was when the
trigger was created. This is also the
character_set_client
that is used when the trig-
ger is invoked.
■
COLLATION_CONNECTION
— Stores the environmental collation as it was when the trig-
ger was created. This is also the
collation_connection
that is used when the trigger is
invoked.
■
DATABASE_COLLATION
— The default collation of the database associated with the
trigger.
For more information on triggers, see Chapter 7.
The
SHOW TRIGGERS
statement outputs some of the same information in the
TRIGGERS
system
view. The equivalent fields are:
■
Trigger
— Equivalent to
TRIGGER_NAME
.
■

Event
— Equivalent to
EVENT_MANIPULATION
.
■
Table
— Equivalent to
EVENT_OBJECT_TABLE
.
■
Statement
— Equivalent to
ACTION_STATEMENT
.
■
Timing
— Equivalent to
ACTION_TIMING
.
■
Created
,
sql_mode
,
Definer
,
character_set_client
,
collation
connection

— Equivalent to
CREATED
,
SQL_MODE
,
DEFINER
,
CHARACTER_SET_CLIENT
,
COLLATION_CONNECTION
.
The
SHOW CREATE TRIGGER
statement outputs the following fields, which are similar to fields in
the
TRIGGERS
system view:
■
Trigger
— Equivalent to
TRIGGER_NAME
.
■
sql_mode
— Equivalent to
SQL_MODE
.
■
SQL Original Statement
— This is the entire

CREATE TRIGGER
statement, which
includes the information shown in the
EVENT_MANIPULATION
,
ACTION_ORDER
,
ACTION_STATEMENT
,
ACTION_TIMING
,and
DEFINER
fields.
■
character_set_client
,
collation connection
,
Database Collation —
Equivalent to
CHARACTER_SET_CLIENT
,
COLLATION_CONNECTION
,and
DATABASE_
COLLATION
.
ROUTINES
The
ROUTINES

system view stores metadata about stored procedures and stored routines.
For more information on stored routines, see Chapter 7. The fields in the
ROUTINES
system
view are:
■
SPECIFIC_NAME
— The name of the stored routine.
■
ROUTINE_CATALOG
— Provided for standards compliance. However, because MySQL
does not have catalogs, this value is always
NULL
.
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