SQL PROGRAMMING STYLE- P24 ppsx

102 CHAPTER 6: CODING CHOICES

1. We build the CROSS JOIN of the two tables. Scan each row in
the result set.
2. If the predicate tests TRUE for that row, then you keep it. You
also remove all rows derived from it from the CROSS JOIN.
3. If the predicate tests FALSE or UNKNOWN for that row, then
keep the columns from the preserved table, convert all the
columns from the unpreserved table to NULLs, and remove the
duplicates.
So let us execute this by hand:

Let @ = passed the first predicate
Let * = passed the second predicate

Table1 CROSS JOIN Table2

a b a c
=========================
1 w 1 r @
1 w 2 s
1 w 3 t *
2 x 1 r
2 x 2 s @
2 x 3 t *
3 y 1 r
3 y 2 s
3 y 3 t @* <== the TRUE set
4 z 1 r
4 z 2 s

4 z 3 t *

Table1 LEFT OUTER JOIN Table2

a b a c
=========================
3 y 3 t <= only TRUE row

1 w NULL NULL Sets of duplicates
1 w NULL NULL
1 w NULL NULL

6.1 Pick Standard Constructions over Proprietary Constructions 103


2 x NULL NULL
2 x NULL NULL
2 x NULL NULL
3 y NULL NULL <== derived from the TRUE set - Remove
3 y NULL NULL

4 z NULL NULL
4 z NULL NULL
4 z NULL NULL=

The final results:

Table1 LEFT OUTER JOIN Table2
a b a c
=========================

1 w NULL NULL
2 x NULL NULL
3 y 3 t
4 z NULL NULL

The basic rule is that every row in the preserved table is represented
in the results in at least one result row.

6.1.1.1 Extended Equality and Proprietary Syntax

Before the standard was set, vendors all had a slightly different syntax
with slightly different semantics. Most of them involved an extended
equality operator based on the original Sybase implementation. There
are limitations and serious problems with the extended equality,
however. Consider the two Chris Date tables:

Suppliers SupParts
supno supno partno qty
========= ==============
S1 S1 P1 100
S2 S1 P2 250
S3 S2 P1 100
S2 P2 250

104 CHAPTER 6: CODING CHOICES

And let’s do a Sybase-style extended equality OUTER JOIN like this:

SELECT *
FROM Supplier, SupParts

WHERE Supplier.supno *= SupParts.supno
AND qty < 200;

If I do the OUTER join first, I get:

Suppliers LOJ SupParts
supno supno partno qty
=======================
S1 S1 P1 100
S1 S1 P2 250
S2 S2 P1 100
S2 S2 P2 250
S3 NULL NULL NULL

Then I apply the (qty < 200) predicate and get:

Suppliers LOJ SupParts
supno supno partno qty
===================
S1 S1 P1 100
S2 S2 P1 100

Doing it in the opposite order results in the following:

Suppliers LOJ SupParts
supno supno partno qty
===================
S1 S1 P1 100
S2 S2 P1 100
S3 NULL NULL NULL

Sybase does it one way, Oracle does it another, and Centura (née
Gupta) lets you pick which one to use—the worst of both nonstandard
worlds! In SQL-92, you have a choice and can force the order of
execution. Either do the predicates after the join:

6.1 Pick Standard Constructions over Proprietary Constructions 105

SELECT *
FROM Supplier
LEFT OUTER JOIN
SupParts
ON Supplier.supno = SupParts.supno
WHERE qty < 200;

or do it in the joining:

SELECT *
FROM Supplier
LEFT OUTER JOIN
SupParts
ON Supplier.supno = SupParts.supno
AND qty < 200;

Another problem is that you cannot show the same table as preserved
and unpreserved in the extended equality version, but it is easy in SQL-
92. For example, to find the students who have taken Math 101 and
might have taken Math 102:

SELECT C1.student, C1.math, C2.math

FROM (SELECT * FROM Courses WHERE math = 101) AS C1
LEFT OUTER JOIN
(SELECT * FROM Courses WHERE math = 102) AS C2
ON C1.student = C2.student;

Exceptions:

None. Almost every vendor, major and minor, has the ANSI infixed
OUTER JOIN operator today. You will see various proprietary notations
in legacy code, and you can convert it by following the discussion given
previously.

6.1.2 Infixed INNER JOIN and CROSS JOIN Syntax Is
Optional, but Nice

SQL-92 introduced the INNER JOIN and CROSS JOIN operators to
match the OUTER JOIN operators and complete the notation; other
infixed JOIN operators are not widely implemented but exist for
completeness. The functionality of the INNER JOIN and CROSS JOIN

106 CHAPTER 6: CODING CHOICES

existed in the FROM clause before and did not give the programmer
anything new like the OUTER JOINs.

Rationale:

The CROSS JOIN is a handy piece of documentation that is much harder
to miss seeing than a simple comma. Likewise, writing out INNER JOIN
instead of the shorthand INNER helps document the code.

However, many INNER JOIN operators can be visually confusing,
and you might consider using the older syntax. The older syntax lets you
put all of the predicates in one place and group them in some manner
for readability. A rule of thumb is the “rule of five” in human
psychology. This says that we have problems handling more than five
things at once, get serious problems with seven, and break down at nine
(Miller 1956).
So when you have fewer than five tables, the infixed operators are fine
but questionable for more than five INNER JOIN-ed tables. Trying to
associate ON clauses to INNER JOIN operators is visually difficult. In
particular, a Star Schema has an easily recognized pattern of joins from
the fact table to each dimension table, like this in pseudocode:

SELECT
FROM Facts, Dim1, Dim2, , DimN
WHERE Facts.a1 = Dim1.a
AND Facts.a2 = Dim2.a

AND Facts.an = DimN.a

The reader can look down the right-hand side of the WHERE clause
and see the dimensions in a vertical list.
One style that is popular is to put the join conditions in the FROM
clause with INNER JOIN syntax, then do the search arguments in the
WHERE clause. Some newbies believe that this is required, but it is not.
However, if the search arguments change, having them in one place is
handy.
A quick heuristic when using old-style joins is that the number of
tables in the FROM clause should be one more than the number of join
conditions in the WHERE clause. This shows that you do not have cycles

in the joins. If the difference between the number of tables and the
number of join conditions is more than one, then you might have an
unwanted CROSS JOIN caused by a missing join condition.
Old style: