Question

We have this statement:

(SELECT res_bev.bev_id, property_value.name AS priority 
    FROM res_bev, bev_property, property_value 
    WHERE res_bev.res_id='$resIn'
        AND bev_property.bev_id=res_bev.bev_id
        AND bev_property.type_id='23'
        AND property_value.id=bev_property.val_id)
UNION
(SELECT res_bev.bev_id, property_value.name as priority 
    FROM res_bev, bev_property, property_value 
    WHERE res_bev.res_id='$resIn' 
        AND bev_property.bev_id=res_bev.bev_id 
        AND bev_property.type_id='22' 
        AND property_value.id=bev_property.val_id)

We have Three Tables:

Res_bev
res_id | bev_id | id

Bev_property
type_id | val_id | bev_id | id

Property_value
name | id

What I am looking for is the results to be ordered by glass price(type_id=’23’) then bottle price(type_id=’22’) however it seems the union includes duplicates due to fact the first select returns say 3456 | 7.5 and the second returns 3456 | 55 since the price/Glass is 7.5 and the price/Bottle is 55; how can I eliminate these duplicates form the second SQL statement to return and ordered table?

Also, fooled with creating a pseudo-table via left joins to create a table of bev_id | price/Glass | price/Bottle, however since this should be able to expand to multiple price types I figured a UNION would be more efficient. Just a push in the right direction would be helpful.

Answer 1

You can do it in 1 query by specifying bev_property.type_id to match against an IN() clause with the values inside.

To return only the first one found you should require a DISTINCT SELECT of the accompagnying field bev_id.

To ORDER them just add an appropriate descending ORDER BY clause. This should order first and the filter out the second bev_property.type_id value. (Databases never return anything in a specific order unless you tell them to, some might have an internal convention or it might appear they do but this is never guaranteed to be repeatable unless you specify an ORDER BY clause in your SELECT statement. )

SELECT DISTINCT res_bev.bev_id, property_value.name AS priority 
FROM res_bev, bev_property, property_value 
WHERE res_bev.res_id='$resIn'
    AND bev_property.bev_id=res_bev.bev_id
    AND bev_property.type_id IN ('23','22')
    AND property_value.id=bev_property.val_id
ORDER BY bev_property.type_id DESC;

A UNION won’t really be faster since you’d have to do the whole lookup twice and if you don’t have this field indexed then you’ll do a whole table traversal with match against 1 element twice as opposed do 1 table traversal that matches against 2 elements. (walking over a whole table is what’s generally slow, not matching simple elements against each other)

When properly indexed I think you might have a tiny overhead of executing a new select query and the query analyzer running again but I don’t know for sure. It’ll probably be smart enough to recognise the similarities between the queries so it won’t matter.

It doesn’t always hurt to try on specific databases though. Whenever you try query optimisation with different statements use them with EXPLAIN, this will show you what the query will be doing and wether it’ll go over whole tables, sort data on file, etc…