Tài liệu Retrieving Column Default Values from SQL Server pdf

[ Team LiB ]Recipe 10.3 Retrieving Column Default Values from SQL Server Problem The DataColumn object exposes a Default property.. While the FillSchema method of the DataAdapter retu

[ Team LiB ]

Recipe 10.3 Retrieving Column Default Values from SQL Server

Problem

The DataColumn object exposes a Default property While the FillSchema( ) method of the DataAdapter returns schema information, it does not include the default values for columns You want to retrieve the default values of columns in a SQL Server table

Solution

Use system stored procedures

The sample code executes the system stored procedure sp_helpconstraint to get constraint information for the columns in the Orders table in the Northwind sample database

Column default values are identified and retrieved from the result set

The C# code is shown in Example 10-3

Example 10-3 File: ColumnDefaultsForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Text;

using System.Data;

using System.Data.SqlClient;

//

StringBuilder result = new StringBuilder( );

// Fill the Orders table with schema and data

SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Orders",

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

DataTable ordersTable = new DataTable(ORDERS_TABLE);

da.FillSchema(ordersTable, SchemaType.Source);

da.Fill(ordersTable);

SqlConnection conn = new SqlConnection(

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

// Command for system stored procedure returning constraints

SqlCommand cmd = new SqlCommand("sp_helpconstraint", conn);

cmd.CommandType = CommandType.StoredProcedure;

cmd.Parameters.Add("@objname",SqlDbType.NVarChar,776);

cmd.Parameters[0].Value = "Orders";

cmd.Parameters.Add("@nomsg",SqlDbType.VarChar,5);

cmd.Parameters[1].Value = "nomsg";

// Create a DataReader from the stored procedure

conn.Open( );

SqlDataReader dr = cmd.ExecuteReader( );

// Iterate over the constraints records in the DataReader

while(dr.Read( ))

{

// Select the default value constraints only

String constraintType = dr["constraint_type"].ToString( );

if (constraintType.StartsWith("DEFAULT"))

{

String constraintKeys = dr["constraint_keys"].ToString( );

// Only strips single quotes for numeric default types

// add necessary handling as required for nonnumeric defaults

String defaultValue =

constraintKeys.Substring(1, constraintKeys.Length - 2);

String colName = constraintType.Substring(

(constraintType.LastIndexOf("column") + 7));

ordersTable.Columns[colName].DefaultValue = defaultValue;

result.Append("Column: " + colName + " Default: " +

defaultValue + Environment.NewLine);

}

}

dr.Close( );

conn.Close( );

resultTextBox.Text = result.ToString( );

Discussion

The default value for a column in SQL Server is stored as a DEFAULT constraint The system stored procedure sp_helpconstraint returns information about all constraints on a

table The procedure takes one mandatory parameter that specifies the table for which to return the constraint information

The first column that the stored procedure returns is called constraint_type As its name suggests, it specifies the type of constraint using the following pattern {constraint_type} [on column {column_name}] For example, the default constraint on the Freight column

in the Orders table in the Northwind sample database has a constraint type of DEFAULT

on column Freight

In the solution, a result set is created from the system stored procedure sp_helpconstraint specifying the Orders table The constraint_type column is examined for each row to determine whether it begins with the word DEFAULT indicating a default constraint For default constraints, the column name is the string following the word column in the constraint_type column

Once the default constraints have been identified, the overloaded constraint_keys column contains the default value for the column The default value is surrounded by parentheses

as well as delimiters for nonnumeric fields—for example, single quotes by default in SQL Server for dates and strings, and an additional prefix N in the case of Unicode strings These delimiters need to be stripped from the value before it can be assigned to the DefaultValue property for the column

For more information about the sp_helpconstraint system stored procedure, see Microsoft SQL Server Books Online

[ Team LiB ]