Question

How can I reuse a computed column in SQL in MySQL? Say: my query is something like: –

SELECT      CONVERT_TZ(         if(timestamp_start > last_update, timestamp_start, last_update),          'GMT',          user.timezone     ) as time_usr_tz from      shecdule     inner join user on shecdule.user_id = user.user_id where      CONVERT_TZ(         if(timestamp_start > last_update, timestamp_start, last_update),          'GMT',          user.timezone     )     < CURRENT_TIMESTAMP(); 

If you see the query the ‘CONVERT_TZ…..’ part is repeated.

This is only a sample query. Actually, I have to use that computed column several times. So if I make change in one place, I have to change in many places. And the size of the query becomes scary, too.

Is there any way to avoid such duplication?

UPDATE I already have sub-query in my original query, so sub-query is not a preferable option. Is it possible in any other way?

Answer 1

SELECT  ctz FROM    (         SELECT  shecdule.*,                 CONVERT_TZ(                 if(timestamp_start > last_update, timestamp_start, last_update),                  'GMT',                  user.timezone                 ) AS ctz         FROM    shecdule         INNER JOIN                 user         ON      user.user_id = s.user_id         ) s WHERE  ctz < CURRENT_TIMESTAMP() 

Note that this query doesn’t use the indexes on timestamp_start and last_update efficiently.

See the performance details in the article in my blog:

• Selecting timestamps for a time zone

In two words, you better use this:

SELECT  ctz FROM    (         SELECT  shecdule.*,                 CONVERT_TZ(                 if(timestamp_start > last_update, timestamp_start, last_update),                  'GMT',                  user.timezone                 ) AS ctz         FROM    shecdule         INNER JOIN                 user         ON      user.user_id = s.user_id         WHERE   timestamp_start < CURRENT_TIMESTAMP() + INTERVAL 14 HOUR                 OR last_update < CURRENT_TIMESTAMP() + INTERVAL 14 HOUR         ) s WHERE  ctz < CURRENT_TIMESTAMP() 

This inner query will perform coarse filtering on timestamp_start and last_update using indexes (no zone conversion can offset the time more than +14:00 hours from UTC).

The outer subquery will then fine filter the results based on user’s time zone.

If you don’t like subqueries, you may also use:

SELECT  shecdule.*,         CONVERT_TZ(         if(timestamp_start > last_update, timestamp_start, last_update),          'GMT',          user.timezone         ) AS ctz FROM    shecdule INNER JOIN        user ON      user.user_id = s.user_id /*          WHERE   timestamp_start < CURRENT_TIMESTAMP() + INTERVAL 14 HOUR                 OR last_update < CURRENT_TIMESTAMP() + INTERVAL 14 HOUR */ HAVING  ctz < CURRENT_TIMESTAMP()