Tài liệu Determining the Length of Columns in a SQL Server Table doc

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Recipe 10.4 Determining the Length of Columns in a SQL Server Table
Problem
The FillSchema( ) method of the DataAdapter returns the correct length in the
MaxLength property for string columns in a SQL Server database, but it returns -1 for the
length of all other fields. You need to get the length of columns other than string type
columns.
Solution
Use the system stored procedure sp_help.
The sample code executes a batch query to return all rows from both the Orders and
Order Details tables in the Northwind sample database. The extended stored procedure
sp_help is used to get the length, precision, and scale of all columns in both tables.
The C# code is shown in Example 10-4
.
Example 10-4. File: ColumnSchemaForm.cs
// Namespaces, variables, and constants
using System;
using System.Configuration;
using System.Collections;
using System.Text;
using System.Data;
using System.Data.SqlClient;

// . . .

StringBuilder schemaInfo = new StringBuilder( );

// Create a batch query to retrieve order and details.
String sqlText = "select OrderID, CustomerID, EmployeeID, OrderDate, " +

"RequiredDate, ShippedDate, ShipVia, Freight, ShipName, " +
" ShipAddress, ShipCity, ShipRegion, ShipPostalCode, ShipCountry " +
"FROM Orders;" +
"SELECT OrderID, ProductID, UnitPrice, Quantity, Discount " +
"FROM [Order Details];";

// Create the connection.
SqlConnection conn = new SqlConnection(
ConfigurationSettings.AppSettings["Sql_ConnectString"]);

// Create DataAdapter.
SqlDataAdapter da = new SqlDataAdapter(sqlText, conn);

// Add table mappings.
da.TableMappings.Add("Table", "Orders");
da.TableMappings.Add("Table1", "Order Details");

// Create the DataSet.
DataSet ds = new DataSet( );
// Fill the schema and data.
da.FillSchema(ds, SchemaType.Mapped);
da.Fill(ds);

// Iterate over the table collection in the DataSet.
foreach(DataTable dt in ds.Tables)
{
schemaInfo.Append("TABLE: " + dt.TableName + Environment.NewLine);

// Create the command to retrieve column information.
cmd = new SqlCommand("sp_help", conn);

cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add("@objname", SqlDbType.NVarChar, 776);
cmd.Parameters[0].Value = dt.TableName;

conn.Open( );
// Create the DataReader from the command.
SqlDataReader dr = cmd.ExecuteReader( );
// Get the second result set containing column information.
dr.NextResult( );

Hashtable colInfo = new Hashtable( );
// Iterate over the second result to retrieve column information.
while(dr.Read( ))
{
colInfo.Add(dr["Column_name"].ToString( ),
"Length = " + dr["Length"] +
"; Precision = " + dr["Prec"] +
"; Scale = " + dr["Scale"]);
}
dr.Close( );
conn.Close( );

// Iterate over the column collection in the table.
foreach(DataColumn col in dt.Columns)
{
// Get column information.
schemaInfo.Append("\tCOLUMN: " + col.ColumnName +
Environment.NewLine);
schemaInfo.Append("\tAllowDBNull: " + col.AllowDBNull +
Environment.NewLine);

schemaInfo.Append("\tAutoIncrement: " + col.AutoIncrement +
Environment.NewLine);
schemaInfo.Append("\tDataType: " + col.DataType +
Environment.NewLine);
schemaInfo.Append("\tMaxLength: " + col.MaxLength +
Environment.NewLine);
schemaInfo.Append("\tUnique: " + col.Unique +
Environment.NewLine);
schemaInfo.Append("\tOther: " + colInfo[col.ColumnName] +
Environment.NewLine);
schemaInfo.Append(Environment.NewLine);
}
schemaInfo.Append(Environment.NewLine);
}

resultTextBox.Text = schemaInfo.ToString( );
Discussion
The system stored procedure sp_help returns, in addition to the length of all fields, the
precision and scale of columns as appropriate.
The sp_help system stored procedure takes one optional parameter. When this parameter
is the name of a table object, the result set returns data including the length, precision,
and scale of each column in columns named Length, Prec, and Scale.
The column length returned by the sp_help is in bytes, meaning that Unicode data types
(nchar, nvarchar, and ntext) report double the value of the MaxLength property of the
column object. In the example, the ShipCountry column of the Orders table reports
MaxLength = 15 (stores a string up to 15 characters long) and Length = 30 (requires 30
characters to store the 15 Unicode characters).
For both the Orders and Order Details tables, the solution adds column length, precision,
and scale data for each row in the sp_help result set to a Hashtable keyed on the column
name. The code then iterates over the collection of columns in the table and displays the

length, precision, and scale for each from the Hashtable. Information from the
FillSchema( ) method of the DataAdapter is also included. The data type and nullable
properties are available using both sp_help and FillSchema( ).
For more information about the sp_help system stored procedure, refer to Microsoft SQL
Server Books Online.
The GetSchemaTable( ) method of the DataReader also returns all column lengths. The
method returns a DataTable containing column metadata for a DataReader, where the
ColumnSize column contains the lengths. For more information about the
GetSchemaTable( ) method, see the discussion for Recipe 5.3
.
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