How to Use CHECK Constraint in SQL

The CHECK constraint is used to limit the values that can be placed in a column. It ensures that all values in a column satisfy a specific condition.

Syntax:

CREATE TABLE table_name (    column_name datatype CHECK (condition),    ...);

Or add a check constraint to an existing table:

ALTER TABLE table_nameADD CONSTRAINT constraint_name CHECK (condition);

Example 1: Create Table with CHECK

CREATE TABLE employees (    employee_id INT PRIMARY KEY,    age INT CHECK (age >= 18),         -- age must be 18 or older    salary DECIMAL(10, 2) CHECK (salary > 0)  -- salary must be positive);

Example 2: Add CHECK Constraint to Existing Table

ALTER TABLE employeesADD CONSTRAINT chk_salary_positive CHECK (salary > 0);

Notes:

• The CHECK condition can use comparison operators (=, <, >, <=, >=, <>), logical operators (AND, OR, NOT), and more.

• When you try to insert or update a value violating the check, the database throws an error.

• Not all database engines enforce CHECK constraints strictly (e.g., older MySQL versions ignore them).

If you want, I can help you write a CHECK constraint for your specific table!