使用连接的SQL请求

Question

I have 2 tables:
circuit(id_circuit, distance)
and
circuit_langue(id_circuit_language, #id_circuit, language, title) .

if I do a join between circuit and circuit_langue , and it's possible that some objects from circuit don't have a circuit_langue,

what i have to do if I want to recuperate objects without circuit_langue ?

Answer 1

By default, the JOIN ( INNER JOIN ) recovers rows that match on both tables.

If you want to recover both the objects with and without a circuit_langue associated you can use a LEFT OUTER JOIN :

SELECT * FROM circuit c LEFT OUTER JOIN circuit_langue cl
ON c.id_circuit = cl.id_circuit

Answer 2

You most likely did INNER JOIN which shows only those records that have a matching row from both sides of the join (tables in this example).

You need a LEFT JOIN to view all circuits even if there is not circuit_langue row associated with it:

select *
from circuit c
left join circuit_langue cl on
  c.id_circuit = cl.id_circuit

If you only need to display records that don't have a corresponding row in langue table you could add a WHERE condition to above query:

select *
from circuit c
left join circuit_langue cl on
  c.id_circuit = cl.id_circuit
where cl.id_circuit is null

Answer 3

There are four main kinds of joins :

• inner join (which is the default)
• left outer join
• right outer join
• full outer join

You've used the INNER JOIN which only returns the matched values in both tables, while you actually need a LEFT OUTER JOIN which returns all rows from the left table, even if there are no matches in the right table.

For reference, the left table is the first one after the FROM keyword.

For further knowledge:

• RIGHT OUTER JOIN : is exactly the opposite of LEFT OUTER JOIN .
• FULL OUTER JOIN :returns rows when there is a match in either one of the tables.