Question

Previous question linky: MySQL Left Joins

Very kindly we got the following code working:

select
      JustACME.Name,
      JustACME.NameCount,
      COALESCE( COUNT( * ), 0 ) as CountFromResultsTable
   from
      ( select a.Name,
               count(*) as NameCount
           from
              acme a
           group by
              a.Name ) JustACME

      LEFT JOIN results r
         on JustACME.Name = r.Name
   group by
      JustACME.Name

The above code works exactly as we need and brings back the following

Name     Count     Total
Tom      10        5
Bon      9         4

In addition to that information being in the acme and results table, we also need to bring back further information. The ‘results’ table is as follows:

Name     q1
Tom      1
Tom      2
Tom      1
Bob      3
Bob      2
Bob      1
Bob      2

I’d like to add to this query, in the talble called results there is another field called q1. This can have a number in it, either 1, 2, or 3 – what I’d like to do is bring back the number of times that 1,2 or 3 appears (count) for each of the Name that also appear in the table. Make sense?

Basically, the output of the query should be something like this:

Name     Count     Total     q1 = 1    q1 = 2     q1 = 3
Bob      9         4         1         2          1
Tom      10        5         4         1          0

I’m probably going out on a limb here but can it be done???

Thanks in advance,

H.

Answer 1

To get just the Name, q1=1, q1=2, q1=3 table from results only you can use:

SELECT Name,
       COUNT(IF(q1=1,1,NULL)) as q1is1, 
       COUNT(IF(q1=2,1,NULL)) as q1is2,
       COUNT(IF(q1=3,1,NULL)) as q1is3
FROM results
GROUP BY Name

This counts up the number of occurences of each result (per Name) — this works because you only want three extra columns and know inadvance which values of q1 you want to have a columns. (If there were say half a million q1 values there’s no way to do this in MySQL other than writing out the COUNT(IF(q1=i,1,0)) AS q1isi half a million times — not exactly ideal).

To combine with your previous query, perhaps something like this (add it in as a JOIN):

select
  JustACME.Name,
  JustACME.NameCount,
  COALESCE( COUNT( * ), 0 ) as CountFromResultsTable,
  r2.q1is1, r2.q1is2, r2.q1is3                     -- <-- grab those extra rows
from
  ( select a.Name,
           count(*) as NameCount
       from
          acme a
       group by
          a.Name ) JustACME

  LEFT JOIN results r
     on JustACME.Name = r.Name
  LEFT JOIN (SELECT Name,                          -- <-- add the new table in
                 COUNT(IF(q1=1,1,NULL)) as q1is1, 
                 COUNT(IF(q1=2,1,NULL)) as q1is2,
                 COUNT(IF(q1=3,1,NULL)) as q1is3
             FROM results
             GROUP BY Name) r2
     ON JustACME.Name = r2.Name
group by
  JustACME.Name

In fact, I’m sure there’s a way to do this without needing to LEFT JOIN on a sub-query, but my mysql kung fu has run out 😛