Question

I’ve the following three tables:

Table A:

id VARCHAR(32) | value VARCHAR(32) | groupId INT
abcdef         | myValue1          | 1
ghijkl         | myValue2          | 2
mnopqr         | myValue3          | 1

Table B:

id VARCHAR(32) | value VARCHAR(32) | userId INT
abcdef         | myValue4          | 1
uvwxyz         | anotherValue      | 1

Table C:

id VARCHAR(32) | someOtherColumns...
abcdef
ghijkl
mnopqr
...
uvwxyz

Table A and B are used for a m:n-association, thus the “id”-column in both tables references the same field (“id”-column in table c).
What I want to do is (for instance)… select all entries in table A where groupId = 1

SELECT * FROM TableA WHERE groupId = 1

and also select all entries in table B where userId = 1

SELECT * FROM TableB WHERE userId = 1

That’s all no problem… but the following makes the select-statement(s) difficult: How can I merge both select-results and replace the value of the first result? For example:

selecting all entries in Table A where groupId = 1 I’ll get abcdef and also mnopqr.
when I select all entries in Table B where userId = 1 I’ll also get abdef (and additionally uvwxyz).

Now, the value of abcdef in Table B should replace the value in the selection result of table A. And the uvwxyz-entry should be added to the result.

Finally I’m looking for a query which produces the following table:

id VARCHAR(32) | value VARCHAR(32)
abcdef         | myValue4           -- myValue1 from the select-statement in tableA should be overwritten
mnopqr         | myValue2           -- from table A
uvwxyz         | anotherValue       -- from table B

I hope anyone know how to do this… thanks in advance for any suggestion! By the way… it would be great if there is any chance to realize this using one single (long) select statement.

Answer 1

Try this:

SELECT * FROM TableB WHERE userId = 1
UNION
SELECT * FROM TableA WHERE groupId = 1 
       and id not in (select id from TableB where userid = 1)