Using Microsoft SQL Server Constraints
June 11, 2003
In this article, I want to tell you about SQL Server constraints: what a constraint is, when it is used and for which purposes. You can find in this article some examples of constraint creation and SQL Server 2000 constraints enhancements.
A constraint is a property assigned to a column or the set of columns in a table that prevents certain types of inconsistent data values from being placed in the column(s). Constraints are used to enforce the data integrity. This ensures the accuracy and reliability of the data in the database. The following categories of the data integrity exist:
Entity Integrity ensures that there are no duplicate rows in a table. Domain Integrity enforces valid entries for a given column by restricting the type, the format, or the range of possible values. Referential integrity ensures that rows cannot be deleted, which are used by other records (for example, corresponding data values between tables will be vital). User-Defined Integrity enforces some specific business rules that do not fall into entity, domain, or referential integrity categories.
Each of these categories of the data integrity can be enforced by the appropriate constraints. Microsoft SQL Server supports the following constraints:
A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should have a primary key constraint to uniquely identify each row and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.
A UNIQUE constraint enforces the uniqueness of the values in a set of columns, so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.
A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.
A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity.
A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints.
You can create constraints when the table is created, as part of the table definition by using the CREATE TABLE statement.
The following example creates a check_sale CHECK constraint on an employee table:
You can add constraints to an existing table by using the ALTER TABLE statement. The following example adds a pk_employee primary key constraint on an employee table:
You can add the primary or unique key constraint into an existing table only when there are no duplicate rows in the table. You can drop constraints in an existing table by using the ALTER TABLE statement. The following example drops the pk_employee primary key constraint in the employee table:
Sometimes you need to perform some actions that require the FOREIGN KEY or CHECK constraints be disabled, for example, your company does not hire foreign employees, you made the appropriate constraint, but the situation changed. Your boss needed to hire a foreign employee, but only this one. In this case, you need to disable the constraint by using the ALTER TABLE statement. After these actions are performed, you can re-enable the FOREIGN KEY and CHECK constraints by using the ALTER TABLE statement.
The following example disables the check_sale constraint in the employee table and enables this constraint later:
SQL Server 2000 Constraints Enhancements
SQL Server 2000 introduces many new features. Some of them fall into the constraints area. Now you can control the actions SQL Server 2000 takes when you attempt to update or delete a key to which existing foreign keys point. You can control it by using the new ON DELETE and ON UPDATE clauses in the REFERENCES clause of the CREATE TABLE and ALTER TABLE statements. For example, in the previous versions of SQL Server if you wanted to do a cascade delete from the referenced table when the appropriate record in the parent table is deleted, you had to create a trigger which executed on delete of the parent table, but now you can simply specify the ON DELETE clause in the REFERENCES clause.
The following example is used to create the Books and the Authors tables and create a foreign key constraint which will perform the cascade delete action, therefore, when a row in the Authors table is deleted, the corresponding rows in the Books are also deleted:
Constraints are the built-in mechanism for enforcing data integrity. Using constraints is preferred to using triggers, rules, and defaults because built-in integrity features use much less overhead and perform faster than the ones you can create. When you write your own code to realize the same actions as the constraints, there is a potential for error in the code, therefore, the built-in constraints are not only faster, but also are more consistent and reliable. So, you should use triggers and rules only when the constraints do not provide all the needed functionality.