MySQL在一个查询中同时选择SUM和同一列的一个特定行-最佳方式？

Question

Well, the question might not seem very smart, since I solve my problem. I'm just curious if there is better way. I wrote this query:

SELECT 
    SUM(`attr`) AS `attrsum`, 
    (SELECT `attr` FROM `table`
     WHERE `id`=XXX AND `specialcondition`=YYY) AS `attr` 
FROM 
    `table` 
WHERE 
    `id`=XXX

It absolutely works and do what I want, but I'm not sure - is it the best way to do this? I mean subquery is still like another query, so I doubt that this is the optimized way to get those data. On the other hand - I have no idea how it can be done better. Or this is the best way and everything is fine in my query?

Answer 1

Assumming that the subquery is retrieving a single row, you can eliminate the subquery using MAX or MIN aggregate function together with CASE expression in this way:

SELECT 
    SUM(`attr`) AS `attrsum`, 
    MAX( CASE WHEN `specialcondition`=YYY THEN `attr` END ) as `attr` 
FROM `table` WHERE `id`=XXX

A speed gain will depend on the number of records in the table for a single ID, if it's only a few hundred records, it will be unnoticeable, for millions of records without an multicolumn index on (id, specialcondition) can be significant.
For the above query, a simple index on id column is sufficient.