Table aliases - short catalog numbers

Have you already noticed the shortcuts

k

and

a

in the queries? These are table aliases - temporary, short catalog numbers that we give the shelves for the duration of a single query.

Why aliases?

When we join tables, the

id

column exists in both. How is the Library to know whether we mean

ksiazki.id

or

autorzy.id

? We must prefix the column name with the table name. Full names, however, are long:

1SELECT ksiazki.tytul, autorzy.nazwisko
2FROM ksiazki
3INNER JOIN autorzy ON ksiazki.autor_id = autorzy.id;

Shorter with aliases

We give an alias right after the table name, writing a letter (or short shortcut) after a space:

1SELECT k.tytul, a.nazwisko
2FROM ksiazki k
3INNER JOIN autorzy a ON k.autor_id = a.id;

• ksiazki k

  - from now on, in this query,

  k

  means

  ksiazki

  .

• autorzy a

  -

  a

  means

  autorzy

  .

Thanks to this,

k.tytul

and

a.nazwisko

are short and unambiguous. An alias works only within one query - it does not change the table's name in the database.

Good practice

By convention we choose aliases so that they resemble the table name:

k

for

ksiazki

,

a

for

autorzy

,

c

for

czytelnicy

,

w

for

wypozyczenia

,

kat

for

kategorie

. When joining three shelves, aliases save your eyes!