Recipe 6.9 Retrieving Constraints from a SQL Server Database

Problem

You need to programmatically define constraints in a DataSet and retrieve constraint information defined in a SQL Server database.

Solution

Use the INFORMATION_SCHEMA views and SQL Server system tables to get information about primary keys, foreign keys, and check constraints.

The sample code contains one event handler:

Get Constraints Button.Click

Uses a SQL select statement to load the specified constraint information—primary key, foreign key, or check constraint—from the INFORMATION_SCHEMA views into a DataTable.

The C# code is shown in Example 6-27.

Example 6-27. File: ConstraintForm.cs

// Namespaces, variables, and constants
using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Data.OleDb;

private const String GETPRIMARYKEYCONSTRAINTS =
    "SELECT tc.CONSTRAINT_NAME, tc.TABLE_NAME, " +
    "kcu.COLUMN_NAME, kcu.ORDINAL_POSITION " +
    "FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS tc " +
    "JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE kcu ON " +
    "tc.CONSTRAINT_NAME=kcu.CONSTRAINT_NAME " +
    "WHERE tc.CONSTRAINT_TYPE='PRIMARY KEY' " +
    "ORDER BY tc.TABLE_NAME, kcu.COLUMN_NAME, kcu.ORDINAL_POSITION";

private const String GETFOREIGNKEYCONSTRAINTS =
    "SELECT rc.CONSTRAINT_NAME, rc.UPDATE_RULE, rc.DELETE_RULE, " +
    "kcuP.TABLE_NAME ParentTable, kcuC.TABLE_NAME ChildTable, " +
    "kcuP.COLUMN_NAME ParentColumn, kcuC.COLUMN_NAME ChildColumn " +
    "FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS rc " +
    "LEFT JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE kcuP ON " +
    "rc.UNIQUE_CONSTRAINT_NAME=kcuP.CONSTRAINT_NAME " +
    "LEFT JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE kcuC ON " +
    "rc.CONSTRAINT_NAME=kcuC.CONSTRAINT_NAME AND " +
    "kcuP.ORDINAL_POSITION=kcuC.ORDINAL_POSITION " +
    "ORDER BY kcuP.TABLE_NAME, kcuC.TABLE_NAME, kcuP.ORDINAL_POSITION";

private const String GETCHECKCONSTRAINTS =
    "SELECT tc.TABLE_NAME, tc.CONSTRAINT_NAME, cc.CHECK_CLAUSE " +
    "FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS tc " +
    "JOIN INFORMATION_SCHEMA.CHECK_CONSTRAINTS cc ON " +
    "tc.CONSTRAINT_NAME=cc.CONSTRAINT_NAME " +
    "WHERE CONSTRAINT_TYPE='CHECK' " +
    "ORDER BY tc.TABLE_NAME, cc.CONSTRAINT_NAME";

//  . . . 

private void getConstraintsButton_Click(object sender, System.EventArgs e)
{
    // Create the DataAdapter to retrieve schema information.
    SqlDataAdapter da = null;
    if (primaryKeyRadioButton.Checked)
        da = new SqlDataAdapter(GETPRIMARYKEYCONSTRAINTS,
            ConfigurationSettings.AppSettings["Sql_ConnectString"]);
    else if (foreignKeyRadioButton.Checked)
        da = new SqlDataAdapter(GETFOREIGNKEYCONSTRAINTS,
            ConfigurationSettings.AppSettings["Sql_ConnectString"]);
    else if (checkRadioButton.Checked)
        da = new SqlDataAdapter(GETCHECKCONSTRAINTS,
            ConfigurationSettings.AppSettings["Sql_ConnectString"]);

    // Create and fill table with schema information.
    DataTable dt = new DataTable( );
    da.Fill(dt);

    // Bind the default view of the table with the grid.
    constraintsDataGrid.DataSource = dt.DefaultView;
}

Discussion

Information schema views were first available in SQL Server 7.0 and later. They provide system-table independent access to SQL Server metadata. The views are based on system tables and provide a layer of abstraction that allows applications to continue to work properly if the system tables change in future releases of SQL Server. Information schema views provide an alternative to using system stored procedures that were previously and are still available. The INFORMATION_SCHEMA views conform to the SQL-92 Standard.

Information schema views are defined within each database in a schema named INFORMATION_SCHEMA. To access the views, specify the fully qualified view name. In the solution, for example, the view containing metadata about the tables in the database is accessed using the following syntax:

INFORMATION_SCHEMA.TABLES

The metadata returned is limited to that which the user has permission to view. Like any other views, information schema views can also be joined in queries or participate in complex queries to extract specific information. For detailed information about the different views available, refer to SQL Server Books Online.

The following three subsections explain how the solution retrieves details about the primary key, the foreign key, and the check constraints in the database. The information schema views that are used in the solution are described in the subsection following those subsections.

Primary key constraints

Primary key information is obtained by querying the TABLE_CONSTRAINTS (Table 6-11) and KEY_COLUMN_USAGE (Table 6-9) information schema views. The views are joined on the CONSTRAINT_NAME field and restricted to constraints with a CONSTRAINT_TYPE of Primary Key. The result set is sorted on the TABLE_NAME, COLUMN_NAME, and ORDINAL_POSITION fields.

SELECT
    tc.CONSTRAINT_NAME,
    tc.TABLE_NAME,
    kcu.COLUMN_NAME,
    kcu.ORDINAL_POSITION
FROM
    INFORMATION_SCHEMA.TABLE_CONSTRAINTS tc JOIN
    INFORMATION_SCHEMA.KEY_COLUMN_USAGE kcu ON
    tc.CONSTRAINT_NAME = kcu.CONSTRAINT_NAME
WHERE
    tc.CONSTRAINT_TYPE = 'PRIMARY KEY'
ORDER BY
    tc.TABLE_NAME,
    kcu.COLUMN_NAME,
    kcu.ORDINAL_POSITION

Foreign key constraints

Foreign key information is obtained by querying the REFERENTIAL_CONSTRAINTS (Table 6-10) and KEY_COLUMN_USAGE (Table 6-9) information schema views. The REFERENTIAL_CONSTRAINTS view is joined to the KEY_COLUMN_USAGE view on the UNIQUE_CONSTRAINT_NAME column to return information about the parent table and its columns. The REFERENTIAL_CONSTRAINTS view is joined again to the KEY_COLUMN_USAGE view on the CONSTRAINT_NAME matching the ORDINAL_POSITION of the parent column to return information about the child table and its columns. The result set is sorted in ascending order on the parent TABLE_NAME, child TABLE_NAME, and parent constraint column ORDINAL_POSITION.

SELECT
    rc.CONSTRAINT_NAME,
    rc.UPDATE_RULE,
    rc.DELETE_RULE,
    kcuP.TABLE_NAME ParentTable,
    kcuC.TABLE_NAME ChildTable,
    kcuP.COLUMN_NAME ParentColumn,
    kcuC.COLUMN_NAME ChildColumn
FROM
    INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS rc LEFT JOIN
    INFORMATION_SCHEMA.KEY_COLUMN_USAGE kcuP ON
    rc.UNIQUE_CONSTRAINT_NAME = kcuP.CONSTRAINT_NAME LEFT JOIN
    INFORMATION_SCHEMA.KEY_COLUMN_USAGE kcuC ON
    rc.CONSTRAINT_NAME = kcuC.CONSTRAINT_NAME AND
    kcuP.ORDINAL_POSITION = kcuC.ORDINAL_POSITION
ORDER BY
    kcuP.TABLE_NAME,
    kcuC.TABLE_NAME,
    kcuP.ORDINAL_POSITION;

Check constraint

Check constraint information is obtained by querying the TABLE_CONSTRAINTS (Table 6-11) and CHECK_CONSTRAINTS (Table 6-8) information schema views. The views are joined on the CONSTRAINT_NAME field and restricted to constraints with a CONSTRAINT_TYPE of CHECK. The result set is sorted on the TABLE_NAME and CONSTRAINT_NAME fields.

SELECT
    tc.TABLE_NAME,
    tc.CONSTRAINT_NAME,
    cc.CHECK_CLAUSE
FROM
    INFORMATION_SCHEMA.TABLE_CONSTRAINTS tc JOIN
    INFORMATION_SCHEMA.CHECK_CONSTRAINTS cc ON
    tc.CONSTRAINT_NAME = cc.CONSTRAINT_NAME
WHERE
    CONSTRAINT_TYPE = 'CHECK';
ORDER BY
    tc.TABLE_NAME,
    cc.CONSTRAINT_NAME

Information schema views used in this solution

The four information schema views used by the solution are described in this subsection.

Table 6-8 describes the CHECK_CONSTRAINTS information schema view that is based on the sysobjects and syscomments system tables. This view contains one row for each check constraint in the database.

Table 6-8. CHECK_CONSTRAINTS information schema view

Column name	Data type	Description
CONSTRAINT_CATALOG	nvarchar(128)	Constraint qualifier
CONSTRAINT_SCHEMA	nvarchar(128)	Constraint owner name
CONSTRAINT_NAME	sysname	Constraint name
CHECK_CLAUSE	nvarchar(4000)	Transact-SQL statement

Table 6-9 describes the KEY_COLUMN_USAGE information schema view that is based on the sysobjects, syscolumns, sysreferences, spt_values, and sysindexes system tables. This view contains one row for each column in the current database that is constrained as a key.

Table 6-9. KEY_COLUMN_USAGE information schema view

Column name	Data type	Description
CONSTRAINT_CATALOG	nvarchar(128)	Constraint qualifier
CONSTRAINT_SCHEMA	nvarchar(128)	Constraint owner name
CONSTRAINT_NAME	nvarchar(128)	Constraint name
TABLE_CATALOG	nvarchar(128)	Table qualifier
TABLE_SCHEMA	nvarchar(128)	Table owner name
TABLE_NAME	nvarchar(128)	Table name
COLUMN_NAME	nvarchar(128)	Column name
ORDINAL_POSITION	int	Column ordinal

Table 6-10 describes the REFERENTIAL_CONSTRAINTS information schema view that is based on the sysobjects, sysreferences, and sysindexes system tables. This view contains one row for each foreign key constraint in the current database.

Table 6-10. REFERENTIAL_CONSTRAINTS information schema view

Column name	Data type	Description
CONSTRAINT_CATALOG	nvarchar(128)	Database name
CONSTRAINT_SCHEMA	nvarchar(128)	Constraint owner name
CONSTRAINT_NAME	sysname	Constraint name
UNIQUE_CONSTRAINT_CATALOG	nvarchar(128)	Unique database name
UNIQUE_CONSTRAINT_SCHEMA	nvarchar(128)	Unique constraint owner name
UNIQUE_CONSTRAINT_NAME	sysname	Unique constraint name
MATCH_OPTION	varchar(7)	Referential constraint-matching condition
UPDATE_RULE	varchar(9)	Action taken if the T-SQL UPDATE statement violates referential integrity defined by the constraint
DELETE_RULE	varchar(9)	Action taken if the T-SQL DELETE statement violates referential integrity defined by the constraint

Table 6-11 describes the TABLE_CONSTRAINTS information schema view that is based on the sysobjects system table. This view contains one row for each table constraint in the current database.

Table 6-11. TABLE_CONSTRAINTS information schema view

Column name	Data type	Description
CONSTRAINT_CATALOG	nvarchar(128)	Constraint name
CONSTRAINT_SCHEMA	nvarchar(128)	Constraint owner
CONSTRAINT_NAME	sysname	Constraint name
TABLE_CATALOG	nvarchar(128)	Database name
TABLE_SCHEMA	nvarchar(128)	Table owner
TABLE_NAME	sysname	Table name
CONSTRAINT_TYPE	varchar(11)	Type of constraint: CHECK, UNIQUE, PRIMARY KEY, or FOREIGN KEY
IS_DEFERRABLE	varchar(2)	Specifies whether constraint checking can be deferred; always returns the value NO.
INITIALLY_DEFERRED	varchar(2)	Specifies whether constraint checking is initially deferred; always returns the value NO