Q125324: INF: Semantics of SELECT DISTINCT with ORDER BY

Article: Q125324
Product(s): Open Database Connectivity (ODBC)
Version(s): WINDOWS:2.0
Operating System(s): 
Keyword(s): 
Last Modified: 12-JUN-2001

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The information in this article applies to:

- Microsoft Open Database Connectivity, version 2.0 
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SUMMARY
=======

The information in this article applies to ODBC Desktop Database Drivers version
2.0. A SELECT clause that has the DISTINCT modifier in the select list and has
an ORDER BY and the ORDER BY clause has columns that are not present in the
select list, causes an error due to ambiguous semantics. The article discusses
why this occurs.

MORE INFORMATION
================

ODBC Desktop Database drivers are single-tier drivers. They are written as a
thin layer on top of the JET database engine. The JET engine is the SQL engine
that Microsoft Access and Microsoft Visual Basic use to get to dBASE, Paradox,
FoxPro, Btrieve, Access, Excel, and Text file formats.

When a SELECT statement is executed, the JET engine always performs an implicit
sort if a DISTINCT modifier is present; an ORDER BY clause causes another sort
to be done, which occurs after all other sorts (if any).

As an extension to ANSI 89 SQL, the JET engine allows you to ORDER BY columns
that do not appear in the SELECT list. Thus, a SELECT statement of the form

     SELECT x FROM test_table
     ORDER BY y

is a valid SQL statement. On this statement, the engine does a sort on the result
set and throws out the columns that are not selected.

However, the semantics of a statement of the form

     SELECT DISTINCT x FROM test_table
     ORDER BY y

are non-deterministic. The engine does an implicit sort for the SELECT DISTINCT.
If it has the pairs (x0,y1),(x0,y2), one of them has to go because of the
DISTINCT x. But which one should go?

The decision that the engine makes will affect where in the sort order (x0)
appears, because the sort for the ORDER BY clause is always done last. Because
of the non-determinism in the semantics of a SELECT statement, when you try to
execute it, you will generate the error:

  [Microsoft][ODBC Microsoft Access 2.0 Driver] ORDER BY clause (y) conflicts
  with DISTINCT
  SQLSTATE = "22005", NativeError = -3015

Note that what you want might be obtained by the following query:

     SELECT DISTINCT x,y FROM test_table
     ORDER BY y

This will cause duplicate values of x that you would not otherwise see had you
not included y in the select list. However, in most cases, this is a very
acceptable workaround.

Additional query words: 2.00.2317 dBASE Paradox Btrieve Access FoxPro Text Excel SQL 3015 Windows NT

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Keywords          :  
Technology        : kbAudDeveloper kbODBCSearch kbODBC200
Version           : WINDOWS:2.0

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