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You use a CHECK condition to ensure that the values in a column satisfy some criteria or rule. These rules or criteria may be required to verify that the data is correct, or they may be more rigid rules that reflect organization policies and procedures. CHECK conditions on individual column values are useful when only a restricted range of values are valid for that column.
Once a CHECK condition is in place, future values are evaluated against the condition before a row is modified. When you update a value that has a check constraint, the constraints for that value and for the rest of the row are checked.
Variables are not allowed in CHECK constraints on columns. Any string starting with @ within a column CHECK constraint is replaced with the name of the column the constraint is on.
If the column data type is a domain, the column inherits any CHECK constraints defined for the domain.
Column CHECK tests fail if the condition returns a value of FALSE. If the condition returns a value of UNKNOWN, the behavior is as though it returns TRUE, and the value is allowed.
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