The Scriptorium - UPDATE and DELETE

Knowledge is alive, so data must be corrected and sometimes deleted. In the Scriptorium scribes corrected scrolls - we do it with the

UPDATE

and

DELETE

statements.

UPDATE - changing values

UPDATE

changes existing rows. After

SET

we give the new values, and

WHERE

points to which rows to correct:

1UPDATE ksiazki SET cena = 19.99 WHERE id = 5;

DELETE - removing rows

DELETE

removes rows that meet a condition:

1DELETE FROM ksiazki WHERE id = 7;

The golden rule: always WHERE!

This is the scribe's most important warning. If you forget the

WHERE

clause:

1UPDATE ksiazki SET cena = 0;   -- changes the price of ALL books!
2DELETE FROM ksiazki;           -- removes ALL books!

Without

WHERE

the statement affects the whole shelf. This is one of the most dangerous mistakes when working with a database. Before you run

UPDATE

or

DELETE

, make sure you have added a condition pointing to the right rows. Caution is the mark of a master!