RIGHT JOIN - the mirror image

The bridge can also be crossed the other way. RIGHT JOIN is the mirror image of

LEFT JOIN

.

What RIGHT JOIN does

A RIGHT JOIN B

keeps all rows from the table on the right side (

B

), and from the left (

A

) it adds only matched rows. Where there is no pair - NULL on the left.

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

This will show all authors, even those who do not yet have a single book on the shelf. The table on the right (

autorzy

) is "protected" - none of its rows will disappear.

RIGHT JOIN is LEFT JOIN reversed

In practice

LEFT JOIN

is used more often. Every

RIGHT JOIN

can be turned into a

LEFT JOIN

by swapping the tables:

1-- these two queries give the same result
2SELECT a.nazwisko, k.tytul
3FROM ksiazki k RIGHT JOIN autorzy a ON k.autor_id = a.id;
4
5SELECT a.nazwisko, k.tytul
6FROM autorzy a LEFT JOIN ksiazki k ON k.autor_id = a.id;

What was on the right we put on the left - and it stays "protected" just the same. That is why many scribes stick to

LEFT JOIN

only, always "protecting" the table written first.

Remember

• LEFT JOIN - protects the table on the left (

  FROM

  ).
• RIGHT JOIN - protects the table on the right (

  JOIN

  ).

Both add NULL where there is no pair. The choice depends only on which side we want to keep in full.