>>>Hi,
>>>
>>>In order to make sure that my SQL Select will work (different from the one I use here as an example) I need to understand how sub-queries work.
>>>
>>>Here is a sample SQL Select:
>>>

>>>select fk_fld, flda, fldb from Table1 where fk_fld IN (select pk_fld from Table2 where <somecondition> 
>>>   group by pk_fld)
>>>

>>>
>>>My question is, does VFP (or SQL server) when executing the above SQL Select executes the sub-query first (one time) and then executes main query checking every record of Table1 to see if value FK_FLD is in sub-query?
>>>
>>>TIA.
>>
>>Yes, this is how it's executed. I prefer using EXISTS subquery, though, instead of IN/NOT IN. It may perform faster than IN subquery.
>
>Thank you. Speed is not that critical to me (in this case). But my SQL Select also has the OR clause and I wanted to be sure that the OR clause will only fire if the records is not in the sub-query. Here is a closer sample of my final query:
>
>

>select fk_fld, flda, fldb from Table1 where fk_fld IN (select pk_fld from Table2 where <somecondition> 
>  group by pk_fld) OR Table1.pk_field = nPkVariable
>

>
>And I want to make sure that the resulting query will have a unique value of FK_FLD in every record.

That's not correct. In SQL Server (not sure about VFP) there is no short-cuts which exist in many other languages. So, SQL Server may evaluate both conditions and it's not going to stop right away when one of the conditions is true.

See blog referenced in MSDN thread (by Gert-Jan) for details.

Here is link for your convenience http://www.xs4all.nl/~gertjans/sql/example6/shortcutting-predicate-evaluation.html