SQL Reference
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Quick Reference

The NOT NULL keywords enforces a column to NOT accept NULL values (a record cannot be inserted/updated without adding a value to that field).

The following ensures that the employee_id, last_name, and first_name columns will NOT accept NULL values when the Employees table is created.

CREATE TABLE Employees (
    employee_id int NOT NULL,
    last_name varchar(255) NOT NULL,
    first_name varchar(255) NOT NULL,
    age int
);

The following ensures that the age column will NOT accept a NULL value when the Employees table already exists.

ALTER TABLE Employees
MODIFY age int NOT NULL;

SQL Notes:

  • Any work being done to modify the structure of a database or delete tables or the the database itself should only be done after making a recent backup
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