Using Server Constraints
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Most production SQL databases use constraints to impose conditions on the possible values for a field. For example, a field may not permit NULL values, may require that its value be unique for that column, or that its values be greater than 0 and less than 150. While you could replicate such conditions in your client applications, client datasets and BDE-enabled datasets offer the ImportedConstraint property to propagate a server's constraints locally.
ImportedConstraint is a read-only property that specifies an SQL clause that limits field values in some manner. For example:
Value > 0 and Value < 100
Do not change the value of ImportedConstraint, except to edit nonstandard or server-specific SQL that has been imported as a comment because it cannot be interpreted by the database engine.
To add additional constraints on the field value, use the CustomConstraint property. Custom constraints are imposed in addition to the imported constraints. If the server constraints change, the value of ImportedConstraint also changed but constraints introduced in the CustomConstraint property persist.
Removing constraints from the ImportedConstraint property will not change the validity of field values that violate those constraints. Removing constraints results in the constraints being checked by the server instead of locally. When constraints are checked locally, the error message supplied as the ConstraintErrorMessage property is displayed when violations are found, instead of displaying an error message from the server.