Question

i have three tables: A, B and C, B is the table necessary to break the many to many relationship:

table A   table B   table C
-------   -------   -------
id        id        id
....      id_A      ...
....      id_C      ...

and i have two multiselect html object two display the results, in the second one i need the intersection of both, that is a simple inner join.

the problem is the first one, i need to display there all items are available, so not the same items could be displayed in both.

in a first time, in order to fill the first multiselect i just do a table A left join with table B where the B.id is null, that was ok:

select * from tableA as A
left join tableB as B on A.id = B.id_A
where B.id_A is null

but after that i realized that there is a problem, because if there another relation AC, this relation will appear in both multiselect, so i tried:

select * from tableA as A
left join tableB as B on A.id = B.id_A
where B.id_A is null or B.id_C != $id

and doesn’t work, i need a way to display en the first multiselect only the available and not repeat options every time i change the id_C

i hope somebody can help me 🙂

Answer 1

try this,

SELECT tableC.field1, tableC.field2
FROM tableC
LEFT JOIN tableB ON tableC.ID = tableB.ID_C
AND tableB.ID_A = $ID_A
WHERE tableB.ID_A IS NULL
GROUP BY tableC.field1, tableC.field2