Which constraints can a column have in SQL?

Constraints are rules the database enforces on every write, so bad data is rejected no matter which application sends it.

• NOT NULL: the column must have a value.
• UNIQUE: no two rows share the same value. NULLs are allowed, and most engines treat them as distinct.
• PRIMARY KEY: identifies a row, and combines NOT NULL with UNIQUE. A table has one.
• FOREIGN KEY: the value must exist in a referenced table, with rules such as ON DELETE CASCADE.
• CHECK: a condition each row must satisfy. MySQL enforces it from 8.0.16.
• DEFAULT: a value used when none is given. Strictly it is not a restriction, but it is declared the same way.

CREATE TABLE products (
  id         BIGINT PRIMARY KEY,
  sku        VARCHAR(40) NOT NULL UNIQUE,
  price      NUMERIC(10,2) NOT NULL CHECK (price >= 0),
  status     VARCHAR(20) NOT NULL DEFAULT 'draft',
  brand_id   BIGINT REFERENCES brands (id)
);

Constraints keep integrity in one place, while checks in application code can be bypassed by a script or another service.

Put rules that must never be broken into constraints, not only into code.