Sub-Selects are a database feature.
e.g. for Oracle you can do a select like in this example
SELECT COL1,
COL2,
(SELECT count (*)
FROM TAB2
WHERE COL3 = TAB1.COL1 ) AS CNT
FROM TAB1
The Sub-Select must not return more than 1 row!
You should give it a name because otherwise powerbuilder may crash.