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From:'Bob Diss' Date:December 10 2002 10:05pm
Subject:Re: Bug in GROUP BY/CASE/MAX?
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RE: Bug in GROUP BY/CASE/MAX?

Well, according to my reading/understanding of SQL ANSI'92,
MAX/MIN/SUM/AVG are supposed to eliminate the NULLs from the
set before doing the appropriate calculation...


Bob Diss, rld@stripped

===================================================================

>Date: Tue, 10 Dec 2002 15:45:03 -0600
>From: gerald_clark <gerald_clark@stripped>
>To: "'Bob Diss'" <RLD@stripped>
>
>What is the MAX of a known and an unknown value?
>It would be unknown or NULL
>
>'Bob Diss' wrote:
>
>>RE: Bug in GROUP BY/CASE/MAX?
>>
>>More info -- it seems that if I change the 'NULL' to ''
>>(i.e. the empty string) in the CASE statements, I get the
>>result I'm looking for.  That is, the query:
>>
>>mysql> select max(case when col = 1 then val else '' end) as color
>>       from t group by row;
>>
>>returns the three rows 'orange', 'yellow', and 'green'.
>>
>>Why is that?  Is something broken when MySQL calculates the
>>aggregate for a column that contains a NULL value?  Is there a doc
>>to explain how an aggregate function should work in the presence
>>of NULL?
>>
>>Bob Diss, rld@stripped
>>
>>===================================================================
>>
>>
>>
>>>Date: Tue, 10 Dec 02 13:57:00 EST
>>>From: "'Bob Diss'" <RLD@stripped>
>>>To: "'MySQL List'" <mysql@stripped>
>>>
>>>Bug in GROUP BY/CASE/MAX?
>>>
>>>I'm seeing a strange result when I group rows and using CASE and
>>>MAX() to select the column I'm interested in (typical pivot-table
>>>operation).  Here's my sample case:
>>>
>>>mysql> -- create table
>>>mysql> create table t (row int not null,
>>>                      col int not null,
>>>                      val varchar(255) not null);
>>>
>>>mysql> -- populate with test records
>>>mysql> insert into t values (1,1,'orange');
>>>mysql> insert into t values (1,2,'large');
>>>mysql> insert into t values (2,1,'yellow');
>>>mysql> insert into t values (2,2,'medium');
>>>mysql> insert into t values (3,1,'green');
>>>mysql> insert into t values (3,2,'small');
>>>
>>>mysql> -- group by row, extract values where col=1
>>>mysql> select max(case when col = 1 then val else null end) as color
>>>      from t group by row;
>>>
>>>MySQL shows the result set as:
>>>
>>>+--------+
>>>| color  |
>>>+--------+
>>>| orange |
>>>| NULL   |
>>>| NULL   |
>>>+--------+
>>>3 rows in set (0.00 sec)
>>>
>>>I would have expected MySQL to group the records into
>>>three sets according to row:
>>> (1,1,'orange')
>>> (1,2,'large')
>>>
>>> (2,1,'yellow')
>>> (2,2,'medium')
>>>
>>> (3,1,'green')
>>> (3,2,'small')
>>>
>>>then, for each set, apply the case expression:
>>> (1,1,'orange')  becomes   'orange'
>>> (1,2,'large')   becomes   NULL
>>>
>>> (2,1,'yellow')  becomes   'yellow'
>>> (2,2,'medium')  becomes   NULL
>>>
>>> (3,1,'green')   becomes   'green'
>>> (3,2,'small')   becomes   NULL
>>>
>>>then, apply the max() aggregate:
>>> max('orange',NULL)   becomes  'orange'
>>> max('yellow',NULL)   becomes  'yellow'
>>> max('green',NULL)    becomes  'green'
>>>
>>>So, I'm expecting to see a result set of three colors. This query
>>>works as I expect in MS-SQLServer 7.0.  Why doesn't it work the same
>>>way in MySQL?
>>>
>>>I'm running the RPMS from MySQL.com for MySQL-max 3.23.52
>>>on RedHat 7.3.
>>>
>>>tia,
>>>
>>>Bob Diss, rld@stripped
>>>
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>>>
>>>
>>
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>>
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Thread
Bug in GROUP BY/CASE/MAX?'Bob Diss'10 Dec
RE: Bug in GROUP BY/CASE/MAX?'Bob Diss'10 Dec
  • Re: Bug in GROUP BY/CASE/MAX?gerald_clark10 Dec
  • Re: Bug in GROUP BY/CASE/MAX?Dan Nelson10 Dec
Re: Bug in GROUP BY/CASE/MAX?'Bob Diss'10 Dec