Unique Constraints and Check Constraints

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 topic contains the following sections.

UNIQUE Constraints

CHECK Constraints

Related Tasks

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 do not 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 is not 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 duplicated values, the Database Engine returns an error and does not 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 has been violated and does not 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.

[Top]

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.

Limitations of CHECK Constraints

CHECK constraints reject values that evaluate to FALSE. Because null values evaluate to UNKNOWN, their presence in expressions may 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 does not return an error.

A CHECK constraint returns TRUE when the condition it is checking is not FALSE for any row in the table. A CHECK constraint works at the row level. If a table that has just been created does not 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 are not validated during DELETE statements. Therefore, executing DELETE statements on tables with certain types of check constraints may 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.

[Top]

Related Tasks

[Top]