How to use check constraints in mysql

The CHECK constraint in MySQL is used to limit the range of data values ​​in the table, using the syntax: ALTER TABLE table_name ADD CONSTRAINT constraint_name CHECK (column_name expression). Advantages include data integrity, performance, and maintainability. Notes are that it only applies to a single column, the expression must return a Boolean value, and the constraint name must be unique.

CHECK constraints in MySQL

In MySQL, CHECK constraints are used to limit the data values ​​in a table. It defines the range of values ​​allowed to be stored in the table by specifying a Boolean expression.

How to use CHECK constraints

To create a CHECK constraint, use the following syntax:

<code class="sql">ALTER TABLE table_name ADD CONSTRAINT constraint_name CHECK (column_name expression);</code>

Where:

• table_name is the name of the table to which constraints are to be added.
• constraint_name is the unique name of the constraint.
• column_name is the name of the column to which the constraint is to be applied.
• expression is a Boolean expression that defines the values ​​allowed to be stored in the column.

How to use CHECK constraint example

For example, you want to create a table named employees that only allows positive numbers. ##salary column, you can use the following constraints:

<code class="sql">ALTER TABLE employees ADD CONSTRAINT positive_salary CHECK (salary > 0);</code>

Advantages of CHECK constraints

• Data integrity: It ensures that only values ​​that meet the constraints can be inserted into the table.
• Performance: It can quickly validate data when inserting, thereby reducing the storage of invalid data.
• Maintainability: It makes column constraints easier to understand and manage.

Notes on CHECK constraints

CHECK constraints can only be applied to a single column.
• Expressions must always return a TRUE or FALSE value.
• The constraint name must be unique in the table.

The above is the detailed content of How to use check constraints in mysql. For more information, please follow other related articles on the PHP Chinese website!