Question

I’ve 3 tables say A,B,C.

Table A has userid column.
Table B has caid column.
Table C has lisid and image columns.

one userid can have one or several caids.
one caid can have one or several lisids.

how do I select a userid which has maximum number of rows with image column as not null (in some lisids image column is blank and in some it has some value).

can someone please help.

Answer 1

Presumably, the ids are spread among the tables in a reasonable fashion. If so, the following should do this:

select b.userid, count(*)
from TableB b join
     TableC c
     on b.caid = c.caid
where c.image is not null
group by b.userid
order by count(*) desc
limit 1

The question in the comments is how you connect TableA to TableB and TableB to TableC. The reasonable approach is to have the userid in TableB and the caid in TableC.

Getting all the rows with the max requires a bit more work. Essentially, you have to join in the above query to get the list

select s.*
from (select b.userid, count(*) as cnt
      from TableB b join
           TableC c
           on b.caid = c.caid
     ) s
     (select count(*) as maxcnt
      from TableB b join
           TableC c
           on b.caid = c.caid
      group by b.userid
      order by count(*) desc
      limit 1
     ) smax
     on s.cnt = smax.cnt

Other databses have a set of functions called window functions/ranking functions that make this sort of query much simpler. Alas, MySQL does not offer these.