Question

MySQL derived table syntax

Asked by: DoctorNash

Experts,

this works:

select * from
(select * from ap4prod) as a
inner join
(select distinct prod from pr7prod) as b
on a. prod = b.prod

but this doesn't - could you please advise correct syntax. the error message is 'you have an error in your SQL syntax'

select m. * from
((select * from ap4prod) as a
inner join
(select distinct prod from pr7prod) as b
on a. prod = b.prod) as m

Thanks!

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Asked On
2007-12-25 at 17:04:19ID23042622
Tags

MySQL

Topic

MySQL Server

Participating Experts
1
Points
20
Comments
5

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Answers

 

by: angelIIIPosted on 2007-12-26 at 02:43:31ID: 20528835

what version of MySQL are you using?
subqueries like that are not possible with MySQL 4.0 or lower...

 

by: DoctorNashPosted on 2007-12-26 at 03:24:47ID: 20528886

The version is 5.0.

BTW, this query also doesn't error:

select m.* from
(select a. * from
(select * from ap4prod) as a
inner join
(select distinct prod from pr7prod) as b
on a. prod = b.prod) as m

 

by: angelIIIPosted on 2007-12-26 at 03:29:16ID: 20528896

version 5 is ok.
now, in you latest syntax, you have solved the problem that you had.

now, you should not make it that complex...
try the below syntax, which will be the most effective, and fast if there is an index on pr7prod on column prod

select a.* 
from ap4prod a
where exists (select null from  pr7prod b where a.prod = b.prod) 
                                              
1:
2:
3:

Select allOpen in new window

 

by: DoctorNashPosted on 2007-12-26 at 03:52:14ID: 20528925

Thanks so much for the validation, and alternate logic. The alternate query is running, but is slower than the other way (there's no index on prod in pr7prod). Am not familiar with the structure of  the alternate query - do you mind explaining briefly please (sure does look impressive). For instance:
-is 'ap4prod a' shorthand for 'ap4prod as a'?
-what is the 'select null from...' doing?
-what is the 'where exists' checking?

 

by: angelIIIPosted on 2007-12-26 at 04:18:54ID: 20528958

>-is 'ap4prod a' shorthand for 'ap4prod as a'?
yes

>-what is the 'select null from...' doing?
pure syntax, as the EXISTS() checks if the subquery returns a row or not, it does not matter what column values are returned...

>-what is the 'where exists' checking?
the exists(<subquery>) function checks if the subquery returns rows or not, and returns true if at least 1 row would be returned, and false if no rows are returned.

now, look at the condition in the <subquery>:
where a.prod = b.prod

as you see, the table from the subquery (pr7prod) is "joined" there with the table from the main query (ap4prod). this technique is called "correlated" subquery.

the main advantage of this method is that the EXISTS() will stop looking one it found a matching row, it does not have to perform the DISTINCT, or find all matching rows.


>The alternate query is running, but is slower than the other way (there's no index on prod in pr7prod).
then, create on:
CREATE INDEX IDX_PR7PROD ON PR7PROD ( prod );

and see the difference.

20120131-EE-VQP-002

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