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Unique constraints and check constraints

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Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

UNIQUE constraints and CHECK constraints are two types of constraints that can be used to enforce data integrity in SQL Server tables. These are important database objects.

This article contains the following sections.

UNIQUE constraints

Constraints are rules that the SQL Server Database Engine enforces for you. For example, you can use UNIQUE constraints to make sure that no duplicate values are entered in specific columns that don't participate in a primary key. Although both a UNIQUE constraint and a PRIMARY KEY constraint enforce uniqueness, use a UNIQUE constraint instead of a PRIMARY KEY constraint when you want to enforce the uniqueness of a column (or combination of columns) that isn't the primary key.

Unlike PRIMARY KEY constraints, UNIQUE constraints allow for the value NULL. However, as with any value participating in a UNIQUE constraint, only one null value is allowed per column. A UNIQUE constraint can be referenced by a FOREIGN KEY constraint.

When a UNIQUE constraint is added to an existing column or columns in the table, by default, the Database Engine examines the existing data in the columns to make sure all values are unique. If a UNIQUE constraint is added to a column that has duplicate values, the Database Engine returns an error and doesn't add the constraint.

The Database Engine automatically creates a UNIQUE index to enforce the uniqueness requirement of the UNIQUE constraint. Therefore, if an attempt to insert a duplicate row is made, the Database Engine returns an error message that states the UNIQUE constraint was violated, and doesn't add the row to the table. Unless a clustered index is explicitly specified, a unique, nonclustered index is created by default to enforce the UNIQUE constraint.

CHECK constraints

CHECK constraints enforce domain integrity by limiting the values that are accepted by one or more columns. You can create a CHECK constraint with any logical (Boolean) expression that returns TRUE or FALSE based on the logical operators. For example, the range of values for a salary column can be limited by creating a CHECK constraint that allows for only data that ranges from $15,000 through $100,000. This prevents salaries from being entered beyond the regular salary range. The logical expression would be the following: salary >= 15000 AND salary <= 100000.

You can apply multiple CHECK constraints to a single column. You can also apply a single CHECK constraint to multiple columns by creating it at the table level. For example, a multiple-column CHECK constraint could be used to confirm that any row with a country_region column value of USA also has a two-character value in the state column. This allows for multiple conditions to be checked in one location.

CHECK constraints are similar to FOREIGN KEY constraints in that they control the values that are put in a column. The difference is in how they determine which values are valid: FOREIGN KEY constraints obtain the list of valid values from another table, while CHECK constraints determine the valid values from a logical expression.

Caution

Constraints that include implicit or explicit data type conversion might cause certain operations to fail. For example, such constraints defined on tables that are sources of partition switching might cause an ALTER TABLE...SWITCH operation to fail. Avoid data type conversion in constraint definitions.

Limitations of CHECK constraints

CHECK constraints reject values that evaluate to FALSE. Because null values evaluate to UNKNOWN, their presence in expressions might override a constraint. For example, suppose you place a constraint on an int column MyColumn specifying that MyColumn can contain only the value 10 (MyColumn=10). If you insert the value NULL into MyColumn, the Database Engine inserts NULL and doesn't return an error.

A CHECK constraint returns TRUE when the condition it is checking isn't FALSE for any row in the table. A CHECK constraint works at the row level. If a table that was created doesn't have any rows, any CHECK constraint on this table is considered valid. This situation can produce unexpected results, as in the following example.

CREATE TABLE CheckTbl (col1 INT, col2 INT);
GO

CREATE FUNCTION CheckFnctn()
RETURNS INT
AS
BEGIN
    DECLARE @retval INT;
    SELECT @retval = COUNT(*)
    FROM CheckTbl;

    RETURN @retval;
END;
GO

ALTER TABLE CheckTbl ADD CONSTRAINT chkRowCount CHECK (dbo.CheckFnctn() >= 1);
GO

The CHECK constraint being added specifies that there must be at least one row in table CheckTbl. However, because there are no rows in the table against which to check the condition of this constraint, the ALTER TABLE statement succeeds.

CHECK constraints aren't validated during DELETE statements. Therefore, executing DELETE statements on tables with certain types of check constraints might produce unexpected results. For example, consider the following statements executed on table CheckTbl.

INSERT INTO CheckTbl VALUES (10, 10);
GO
DELETE CheckTbl WHERE col1 = 10;

The DELETE statement succeeds, even though the CHECK constraint specifies that table CheckTbl must have at least 1 row.

Note

If the table is published for replication, you must make schema changes using the Transact-SQL statement ALTER TABLE or SQL Server Management Objects (SMO). When schema changes are made using the Table Designer or the Database Diagram Designer, it attempts to drop and recreate the table. You can't drop published objects, therefore the schema change will fail.

Task Article
Describes how to create a unique constraint. Create Unique constraints
Describes how to modify a unique constraint. Modify Unique Constraints
Describes how to delete a unique constraint. Delete Unique Constraints
Describes how to create a check constraint. Create Check Constraints
Describes how to disable a check constraint when a replication agent inserts or updates data in your table. Disable Check Constraints for Replication
Describes how to disable a check constraint when data is added to, updated in, or deleted from a table. Disable Check Constraints with INSERT and UPDATE Statements
Describes how to change the constraint expression or the options that enable or disable the constraint for specific conditions. Modify Check Constraints
Describes how to delete a check constraint. Delete Check Constraints
Describes how to view the properties of a check constraint. Unique Constraints and Check Constraints