MySQL SELECT JOIN

Question

I tried to get some data from a database, using MySQL JOIN, but with no success until now

I have table_a (representing order lines):

id  idc cPos
1   1   10
2   1   20
3   1   30
4   1   40
5   1   50
6   2   10
7   2   20
8   2   30
9   2   40
10  3   10
11  3   20
12  3   30
13  3   40
14  3   50

And table_b:

delivery_id idc cPos
1           1   10
1           1   50
2           1   20
2           3   40
2           3   50

I would like a query with the below results:

id  idc cPos    delivery_id
1   1   10      1
3   1   30      NULL
4   1   40      NULL
5   1   50      1
6   2   10      NULL
7   2   20      NULL
8   2   30      NULL
9   2   40      NULL
10  3   10      NULL
11  3   20      NULL
12  3   30      NULL

I need all records from table_b where delivery_id equals 1 and all records from table_a that don't have a corespondent in table_b (by corespondent I understand table_a.idc = table_b.idc AND table_a.cPos = table_b.cPos)

The query used by me:

SELECT table_a.*, table_b.delivery_id FROM table_a
LEFT JOIN table_b ON (table_a.idc = table_b.idc AND table_a.cPos = table_b.cPos)
WHERE (delivery_id IS NULL OR delivery_id = 1)

Could someone help me?

Answer 1

You have to create LEFT JOIN with 2 conditions:

SELECT tab_a.*,tab_b.delivery_id
FROM table_a tab_a
LEFT JOIN table_b tab_b ON tab_a.idc=tab_b.idc, tab_a.cPos=tab_b.cPos