Retrieving Constraints from a SQL Server Database

[ Team LiB ] 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

[ Team LiB ]