从mySQL中的单个表行中选择两行

Question

I have a table A (Id1, Id2, someValue)

What I want to achieve in mySQL:

My SELECT query should return two rows:

Id1, someValue
Id2, someValue

(based on a certain condition - for example, where someValue > N)

How can I achieve this without using UNION ?

Answer 1

Since this is not possible without using UNION , here is how to archive this with it:

(SELECT ID1 as ID, someValue FROM A WHERE someValue > N)
UNION
(SELECT ID2 as ID, someValue FROM A WHERE someValue > N)

Answer 2

How about:

SELECT 
    CASE
        WHEN somecondition THEN ID1
        ELSE ID2
    END AS ID, someValue
FROM that_table

The somecondition can be an expression similar to those that you use in a WHERE clause.

Answer 3

Could you not just use an or? for example:

select * from A where someValue = 'x' OR someValue 'y'

Answer 4

If you need the return in separated rows you'd better split the db schema to "id, someValue". On insert, just insert {id1_value, someValue}, {id2_value, someValue}, and on retrieve just select from table where value = someValue. You'll get the return you expect.

If you need to relate id1 and id2 in some way (make a linked list or whatever), you may add the related id to the entry. The schema could be {id, value, related_id}.