[vox-tech] MySQL 3.x subqueries question

[David Hummel]

On Fri, Jan 02, 2004 at 08:12:27PM -0800, David Siedband wrote:
> I have two SQL queries that I'm trying to combine using subqueries.
[snip]
> FWIW, I'm using MySQL 3.23

You need at least MySQL 4.1 for subqueries.
 
> [Parent]
> select distinct Hypoth.ID , Hypoth.ShortName , Hypoth.Priority
> from Hypoth , HypSubCats
> where HypSubCats.SubCatID = <dtml-sqlvar SubCatID type=int>
> and HypSubCats.HypID = Hypoth.ID
> and Hypoth.ShortName !='';
> 
> [child]
> select avg(Priority) as Priority
> from HypImpact
> where HypID = <dtml-sqlvar hid type=int>;
> 
> (note: the hid variable is returned by the first query)
> 
> Here is the concept for the combined query:
> 
> select distinct Hypoth.ID , Hypoth.ShortName , Hypoth.Priority (select 
> avg(Priority) as Priority from HypImpact where HypID = Hypoth.ID)
> from Hypoth , HypSubCats
> where HypSubCats.SubCatID = <dtml-sqlvar SubCatID type=int>
> and HypSubCats.HypID = Hypoth.ID
> and Hypoth.ShortName !='';

Is Priority a column in your Hypoth table?

Your scalar subquery should probably be written like this:

 select
  Hypoth.ID,
  Hypoth.ShortName,
  (select avg(Priority) from HypImpact where HypID = Hypoth.ID) as Priority
 from ...

But you don't need to use a subquery here.  I think the query can be
rewritten as follows:

 select
  Hypoth.ID,
  Hypoth.ShortName,
  avg(HypImpact.Priority) as Priority
 from
  Hypoth
  left join HypSubCats on HypSubCats.HypID = Hypoth.ID
  left join HypImpact on HypImpact.HypID = Hypoth.ID
 where HypSubCats.SubCatID = <whatever> and Hypoth.ShortName != ''
 group by Hypoth.ID, Hypoth.ShortName
 order by Priority

-David