SQL CHECK

CHECK rejects rows where a boolean expression is false. It lets you enforce domain rules without writing a trigger or relying on app code.

Column-level CHECK

CREATE TABLE products (
  id    INT PRIMARY KEY,
  price DECIMAL(10,2) CHECK (price >= 0),
  stock INT           CHECK (stock >= 0)
);

Table-level CHECK (can reference multiple columns)

CREATE TABLE bookings (
  id         INT PRIMARY KEY,
  start_at   DATETIME,
  end_at     DATETIME,
  CONSTRAINT chk_dates CHECK (end_at > start_at)
);

Add or drop later

ALTER TABLE products
ADD CONSTRAINT chk_price CHECK (price >= 0);

ALTER TABLE products
DROP CONSTRAINT chk_price;

CHECK and NULL

A check passes when the expression is TRUE or NULL — only an explicit FALSE rejects the row. CHECK (price >= 0) allows price = NULL. Combine with NOT NULL if you want both.

MySQL note

MySQL accepted CHECK syntax for years without enforcing it. As of MySQL 8.0.16+ it does. Older MariaDB versions may still ignore them — check your server.

Example

ALTER TABLE products
ADD CONSTRAINT chk_price CHECK (price >= 0);

Exercise

Test yourself