SQL NOT NULL Constraint

By default, in SQL a column can accept NULL values.

The NOT NULL constraint specifies that a column does not accept NULL values.

If a NOT NULL constraint is specified on a column, then SQL will not accept to insert a new row in a table without adding a non-NULL for that column.

Note: NULL means no entry has been made. It is different from a blank, zero (0), or a zero-length character string such as ''.

SQL NOT NULL on CREATE TABLE

The following SQL statement will create a new table called "Employee" with five columns, out of which three columns, id, first_name, last_name, do not accept NULL values.

CREATE TABLE employee (
    id INT NOT NULL
    first_name VARCHAR(40) NOT NULL,
    last_name VARCHAR(40) NOT NULL,
    birth_data DATE,
    hire_date DATE
);

SQL NOT NULL on ALTER TABLE

The following SQL statement will add a NOT NULL constraint on the "birth_date" column after the creation of the "Employee" table.

ALTER TABLE employee
    MODIFY birth_date DATE NOT NULL;