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First result only from a subquery.

Hi all,

Here's an MS-SQL subquery I need to duplicate under Oracle:

(SELECT TOP 1 AField FROM ATable WHERE (OtherTable.A = A) AND (OtherTable.B = B) AND (OtherTable.C = C)) AS Result

Am I right in assuming that this will be equivalent:

(SELECT AField FROM ATable WHERE ( ROWNUM = 1 ) AND (OtherTable.A = A) AND (OtherTable.B = B) AND (OtherTable.C = C)) AS Result

or should it be:

(SELECT AField FROM Table WHERE (OtherTable.A = A) AND (OtherTable.B = B) AND (OtherTable.C = C) AND (ROWNUM = 1)) AS Result

or is there no difference or will this not work? If the latter, how can I make it work?

A speedy response would be greatly appreciated. :-)

1 Solution
Ivo StoykovCommented:
Hi PaulCaswell,

in general there is equal how you will write where clause. Oracle optimiser will choose the best way.
inner brackets are not necessary because you're using AND

a potential problem is that there is no join (if necessary at all) between Table and OtherTable...



If Your query doesn't have GROUP BY / ORDER BY clause You can make ROWNUM = 1 part of Your original query - otherwise You have to put it outside.
Btw. order of predicates doesn't matter, so both of queries are identical. All others (without rownum) will be evaluated, rows will be numbered and then ROWNUM = 1 applied.
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Mark GeerlingsDatabase AdministratorCommented:
I don't know MS-SQL, so I don't completely understand the MS-SQL syntax.  In pseudo-code, is this what you are trying to do:
select the highest value from fieldA in tableA
where tableA, fieldA = some valueA
and tableA, fieldB = some valueB
and tableA, fieldC = some valueC ?

I also want to give you a couple cautions about Oracle.  First, Oracle has no "top" command.  Oracle does support "max" (and "min" and other group operators).  Second, Oracle does have "rownum", but you have to be *VERY* careful with using that, since it is applied as rows are retrieved (which is often, but not always in the order they were inserted) but *BEFORE* the rows are sorted based on an "order by" clause, or a "max" or other group operator.  It is possible in Oracle to use a nested select (a type of sub-query which includes an "order by", or a group operator like: "max") then use rownum in the outer select to retrieve the first (top) row from the nested select.
PaulCaswellAuthor Commented:
Thanks people.

Just to confirm: ATable and OtherTable share columns A, B and C. My main query is on OtherTable. I need to find the record in ATable that has the same values of A, B, and C as in OtherTable and pull the AField from it but occasionally (very rarely) there can be two such records in ATable. Obviously this will cause an error so I must avoid it.

I need to be confident here as this situation might only happen a few times a year and this query may be executed many times per day.

>>Btw. order of predicates doesn't matter, so both of queries are identical. All others (without rownum) will be evaluated, rows will be numbered and then ROWNUM = 1 applied.
I think this answers my question. Am I right?

If Your main concern was if this matters (I mean - if this is evaluated in the moment You expect it to be, after other conditions) - then yes.

General truth about migrating TOP N queries from SQL Server to Oracle is in my first post. Those examples are semantically equivalent.
PaulCaswellAuthor Commented:
Thanks! It worked! :-)
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