Question

I need to select all IDs from one table with columns ID and X, WHERE X = 'Y'. For each of those IDs, I need to look up some stuff in a different table:

If the ID does not exist, it gets no row in the final result.

If the ID does exist, I want to do some logic to figure out if it gets a row. For simplicity, assume that the logic is: if column Q > 0.

So the final result is simply a column of IDs, throwing out some because they are disqualified for one of two reasons as above.

thanks.

Answer 1

This is what JOINs are made for.

SELECT table1.* FROM table1
INNER JOIN table2 
    ON table1.ID = table2.table1_ID
    AND table2.Q > 0;

This will select all records in table1 (which have IDs) and then remove any records that do not have a matching record in table2 or do not have a Q > 0.