Question

Subquery returning more than one row

Asked by: grahamtilson

I'm using a subquery which could return more than one row.  At the moment, I force it to only return one row with the limit clause to make sure the query always works.  However......

I'm a convert to mysql from sql server and on the whole I've found mysql far superior, but sql server does have a really useful feature - FOR XML which converts a rowset into an XML string which you can output as  a normal column.  This allows the subquery to return more than one row.

Is there any way to emulate this in mysql?

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Asked On
2008-07-31 at 03:58:29ID23610450
Tags

mysql

Topic

MySQL Server

Participating Experts
1
Points
500
Comments
6

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Answers

 

by: virmaiorPosted on 2008-07-31 at 06:29:27ID: 22129444

(1) MySQL has rather limited XML support (http://dev.mysql.com/doc/refman/5.1/en/xml-functions.html).  It seems like you have to manually create the XML yourself...  

(2) The subquery returning more than one row means that your data is not normalized or that a subquery for a value is really not the appropriate way to organize your data.

 

by: grahamtilsonPosted on 2008-07-31 at 07:05:03ID: 22129783

To be clear, I don't require XML, I really don't care what format the data is in as I can easily deal with that after the query, I just need the subquery to return more than one row as is possible in sql server using the FOR XML syntax.

This is not a showstopper for me as normally the columns wouldn't be multi row and if they were, returning the first one would be acceptable, but it would be a nice to have.

I gather your answer can be summarised as "You can't do it" which was my assessment!"

 

by: virmaiorPosted on 2008-07-31 at 07:08:36ID: 22129811

you can't do it as XML.

there are ways to handle multiple values from a Subquery.

Could you better explain what you are doing / and/or provide the SQL query you are working on?

 

by: grahamtilsonPosted on 2008-07-31 at 08:25:15ID: 22130575

My application allows non technical users to define their own data structure and save documents can contain the fields they have defined.

My query displays a view (also defined by the user) of the documents which looks something like.....

select  id1,

(select text from ......limit 0,1) as surname,
(select text from ......limit 0,1) as first_name,
(select text from ......limit 0,1) as fav_colours
from .......

Which will return something like......

surname                            first_name                        fav_colours
Tilson                                    Graham                                Red
Jones                                    Fred                                         Blue

But I'd like to be able remove the limit and return something like.......

surname                            first_name                        fav_colours
Tilson                                    Graham                                Red, Green, Purple
Jones                                    Fred                                         Blue, Green

or

surname                            first_name                        fav_colours
Tilson                                    Graham                               <colours><colour value="Red"/><colour value="Green"/><colour value="Purple"/></colours>
Jones                                    Fred                                        <colours><colour value="Blue"/><colour value="Green"/></colours>

I can then process the  XML or CSVs in my application.

 

by: virmaiorPosted on 2008-07-31 at 08:27:12ID: 22130596

use GROUP_CONCAT

select  id1,

(select GROUP_CONCAT(text) from ......) as surname,
(select GROUP_CONCAT(text)  from ......) as first_name,
(select GROUP_CONCAT(text)  from ......) as fav_colours
from .......




http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html

 

by: grahamtilsonPosted on 2008-07-31 at 09:36:16ID: 22131277

Hey, you're an absolute star, exactly what I need.  Now why didn't I find that????

Many thanks for your help, you're a guru indeed!

20120131-EE-VQP-002

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