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How to: Define Check Constraint Expressions (Visual Database Tools)

When you attach a check constraint to a table or column, you must include an SQL expression. For details about this operation, see How to: Attach a New Check Constraint to a Table or Column (Visual Database Tools).

You can create a simple constraint expression to check data for a simple condition; or you can create a complex expression, using Boolean operators, to check data for several conditions. For example, suppose the authors table has a zip column where a 5-digit character string is required. This sample constraint expression guarantees that only 5-digit numbers are allowed:

zip LIKE '[0-9][0-9][0-9][0-9][0-9]'

Or suppose the sales table has a column called qty which requires a value greater than 0. This sample constraint guarantees that only positive values are allowed:

qty > 0

To define a constraint expression

  1. Create a new check constraint. For details on how to do this, see How to: Attach a New Check Constraint to a Table or Column (Visual Database Tools).

  2. In the Check Constraints dialog box, type an expression in the Check Constraint Expression dialog box using the following syntax:

    {constant
column_name
function
(subquery)}
[{operator
AND
OR
NOT}
{constant
column_name
function
(subquery)}]

    The SQL syntax is made up of the following parameters:

    Parameter

    Description

    constant

    A literal value, such as numeric or character data. Character data must be enclosed within single quotation marks (').

    column_name

    Specifies a column.

    function

    A built-in function. For details about functions, see System Functions (Transact-SQL).

    operator

    An arithmetic, bitwise, comparison, or string operator. For details about operators, see Using Operators in Expressions.

    AND

    Use in Boolean expressions to connect two expressions. Results are returned when both expressions are true.

    When AND and OR are both used in a statement, AND is processed first. You can change the order of execution by using parentheses.

    OR

    Use in Boolean expressions to connect two or more conditions. Results are returned when either condition is true.

    When AND and OR are both used in a statement, OR is evaluated after AND. You can change the order of execution by using parentheses.

    NOT

    Negates any Boolean expression (which can include keywords, such as LIKE, NULL, BETWEEN, IN, and EXISTS).

    When more than one logical operator is used in a statement, NOT is processed first. You can change the order of execution by using parentheses.