SQL CHECK Keyword

 The CHECK keyword in SQL is used to define a constraint that limits the values that can be inserted into a column. It ensures that the values in a column meet a specified condition. If the condition is not met, an error is raised, and the operation is rejected.

Syntax:

CREATE TABLE table_name (
    column_name data_type,
    CONSTRAINT constraint_name CHECK (condition)
);

Example:

1. Creating a table with a CHECK constraint:

   CREATE TABLE employees (
       id INT,
       name VARCHAR(100),
       age INT,
       CONSTRAINT chk_age CHECK (age >= 18)
   );
   

   In this example, the CHECK constraint ensures that only values where age is greater than or equal to 18 can be inserted into the age column.

2. Using CHECK with a column definition:

   CREATE TABLE students (
       id INT,
       name VARCHAR(100),
       grade CHAR(1),
       CHECK (grade IN ('A', 'B', 'C', 'D', 'F'))
   );
   

   In this example, the CHECK constraint ensures that the grade column only contains values from the set ('A', 'B', 'C', 'D', 'F').

3. Adding a CHECK constraint to an existing table:

   ALTER TABLE employees
   ADD CONSTRAINT chk_age CHECK (age >= 18);
   

Key Points:

• The condition inside the CHECK constraint can reference multiple columns and use operators like =, >, <, BETWEEN, IN, etc.
• It can also be used to ensure that values fall within a certain range or match specific patterns (e.g., an email address).
• If a value violating the CHECK constraint is inserted, an error is thrown, and the insertion is blocked.

Note: Not all database systems support CHECK constraints, or they might implement them with slight variations.