How to: Define Check Constraint Expressions
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.
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
Note
Some databases have different functionality for check constraints. Consult your database documentation for details about how your database works with check constraints.
Note
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To define a constraint expression
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.
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)}]Note
This syntax is Microsoft® SQL Server specific. Consult your database documentation for details.
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 the SQL Server documentation.
operator
An arithmetic, bitwise, comparison, or string operator. For details about operators, see the SQL Server documentation.
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.