Question

I have a mysql database, I want to select all values that are equal on “name” and “postcode”. And the query needs to select the most common data in the other fields.

If I have:

name postcode test  test2
a    a        1     2
a    a        1     2
a    a        2     1
a    a        1     1
a    a        1     1

Then this needs to return

a    a        1     1

Because (test)1 is 4 times in the table, and (test2)1 is there 3 times. So I need the most common data in the Column where the name and the postcode is the same.

Answer 1

This is my first approach:

select distinct
    name, 
    postcode, 
    (select 
       s.test
    from 
       your_table s
    where
       name s.name = m.name, s.postcode = m.postcode
    group by 
       s.name, s.postcode, s.test
    order by count(*) desc
    limit 1 ) as test,
    (select 
       s.test2
    from 
       your_table s
    where
       name s.name = m.name, s.postcode = m.postcode
    group by 
       s.name, s.postcode, s.test2
    order by  count(*) desc
    limit 1 ) as test2
from your_table m

If you don’t need high performance this is a solution. If this query is often performed then you should look for another approach.

EDITED

If you need more performance and also you need distinct rows, you can remove distinct and append group by name, postcode clause at the end of the query.
Query looks like:

select ... group by name, postcode

This is not standard SQL but mysql allow this for better performance:

Quoting MySQL doc:

In standard SQL, a query that includes a GROUP BY clause cannot refer
to nonaggregated columns in the select list that are not named in the
GROUP BY clause. MySQL extends the use of GROUP BY so that the select
list can refer to nonaggregated columns not named in the GROUP BY
clause. This means that the preceding query is legal in MySQL. You can
use this feature to get better performance by avoiding unnecessary
column sorting and grouping.