Question

I have to database tables, where entities of the first Table may or may not have associated entries in the second table:

Table 1          Table 2       
+-----+-----+    +-----+-------+-------+
| ID  | ... |    | ID  | T1_ID | NAME  |
+-----+-----+    +-----+-------+-------+
|  1  | ... |    |  1  |   1   |  p1   |
|  2  | ... |    |  2  |   1   |  p2   |
|  3  | ... |    |  3  |   2   |  p1   |
|  4  | ... |    +-----+-------+-------+
+-----+-----+

I have the following queries i need to run:

1. Get all entities of Table_1 with a specific entry of Table_2 – That’s easy, a simple Join will do…

2. Get all entities of Table_1, which don’t have a specific entry of Table_2 associated – not so easy, but i also managed to query this with a join.

3. Get all entities of Table_1, which have a specific entry (A) and don’t have another specific entry (B) associated, i.e. get all entities of Table_1 that have an entity of Table_2 with name=p1 and don’t have an entity of Table_2 with name=p2 associated.

Is it possible to accomplish the kind of query from (3) in a single sql-statement without a sub-query?

Answer 1

You can use an EXISTS subquery (effectively the same as doing two joins).

SELECT * FROM Table_1 AS t1
WHERE EXISTS (SELECT * FROM Table_2 AS t2 WHERE t1.Id = t2.Id AND Name='p1')
AND NOT EXISTS (SELECT * FROM Table_2 AS t2 WHERE t1.Id = t2.Id AND Name='p2')