Emptiness in the catalog: IS NULL

Not every scroll has all its fields filled in. Sometimes an author's year of birth is unknown, and a borrowed book has not yet been returned. This emptiness - the absence of any value - is called NULL in a database.

NULL is not zero!

Important, @name:

NULL

''

NULL

Why = NULL does not work

Because of this "unknown" nature, we cannot compare

NULL

=

1SELECT * FROM wypozyczenia
2WHERE data_zwrotu = NULL;

The Library does not know whether "unknown" equals "unknown", so it will always return nothing. That is why MySQL has special operators.

IS NULL and IS NOT NULL

To check whether a value is empty, we use IS NULL:

1SELECT * FROM wypozyczenia
2WHERE data_zwrotu IS NULL;

This means: "Loans that have not yet been returned" - because as long as the book is with the reader,

data_zwrotu

To find rows that do have a value, we use IS NOT NULL:

1SELECT * FROM wypozyczenia
2WHERE data_zwrotu IS NOT NULL;

This means: "Loans already returned" - because

data_zwrotu

Remember a simple rule: with NULL we never use

=

IS NULL

IS NOT NULL