Using "OR" in WHERE section of SELECT statement

This is rather idle (as the posted code does solve the problem equally well).

However, you can do it in a single select clause (although a compound one). However, this query depends on non-standard SQL, so the query would not be portable from one DBMS to another. On the other hand it has the advantage of allowing as much values for columnA as you want (the code posted by Phil would require adding additional if statements for each different value you need to test for in columnA if you need more than two values). It is however not very fast if the inner select fetches many rows (the grouping and ordering could be quite expensive, a distinct clause should work fine, also).

Here's how, in MS SQL idiom:

select * from table
where columnA in (
select top 1 columnA
from table
where columnA in (0,x,y,z)
group by columnA
order by columnA)

For Oracle, notice that the "TOP" clause disappears and is replaced by a "HAVING" condition:

select * from table
where columnA in (
select columnA
from table
where columnA in (0, x, y, z)
group by columnA
order by columnA having rownum < 2)

Yet again, another variation (this time, for PostgreSQL):

select * from table
where columnA in (
select columnA
from table
where columnA in (0, x, y, z)
group by columnA
order by columnA limit 1)

As you see, each DBMS requires a different approach, as not all of them implement the same keywords.

Cheers.