MySQL查询优化无子查询

Question

Can anyone help me to optimize this query?

SELECT distinct t.designation,
      (SELECT sum(time_to_sec(timediff(outtime,intime))) 
       FROM onlinetime where datecreated BETWEEN '2012-01-01 00:00:01' 
       AND '2012-12-31 23:59:59' and designation=t.designation) as totsec
FROM onlinetime t;

I am trying to fetch designation and sum of seconds from onlinetime table. It has only 8000 records but the above query taking lot of time to execute. Is there any other way to optimize this?

Answer 1

why do you need to join the table when you can directly get their total summary for each designation

SELECT  designation, sum(time_to_sec(timediff(outtime,intime))) totsec
FROM    onlinetime
WHERE   datecreated BETWEEN '2012-01-01 00:00:01' AND '2012-12-31 23:59:59'
GROUP   BY designation

just add an index on column datecreated for faster performance.

ALTER TABLE onlinetime ADD INDEX (datecreated)

or maybe you want this,

SELECT  a.designation, 
        COALESCE(sum(time_to_sec(timediff(b.outtime,b.intime))), 0) totsec
FROM    onlinetime a
        LEFT JOIN onlinetime b
            ON  a.designation = b.designation AND
                b.datecreated BETWEEN '2012-01-01 00:00:01' AND '2012-12-31 23:59:59'
GROUP   BY a.designation

Answer 2

The correct answer is:

SELECT t.designation,
       sum(case when datecreated between '2012-01-01 00:00:01' AND '2012-12-31 23:59:59'
                then sum(time_to_sec(timediff(outtime,intime)
           end) as totsec
from onlinetime t
group by t.designation

Putting the filtering in the where clause misses the intention of the original query, which is to keep all "designation"s in the table.