Question

Need some help from a MySQL expert here. I have a terrible database to work with and I’m trying to fix the structure a bit but this one has me baffled. The table initially had an id, name and 4 sell columns. I converted that to an id, name and single sell column as basically a pivot table. That was fine, next issue was to get rid of duplicates since not every entry had 4 sell entries.

So after the first operation I ended up with something like this:

id     name     sellid
1      bob      111
1      bob      
1      bob      
2      mary     112
2      mary     113
2      mary     114
2      mary     115
3      fred
3      fred
3      fred
3      fred

So by doing group by I managed to get it to the point where it looks like this:

id     name     sellid
1      bob      111
1      bob      
2      mary     112
2      mary     113
2      mary     114
2      mary     115
3      fred

Now here is where I hit a wall. Fred is fine, he is supposed to have an entry but no sellid, Mary is also fine she has all 4 sellids full. Bob is the issue. How do I remove the empty sellid for him without affecting Fred?

I’d say what I tried but I am just at a complete loss here so I really haven’t tried anything yet.

Answer 1

You are looking for an outer join between your names and other data:

SELECT * FROM
  (SELECT DISTINCT id, name FROM my_table) t1 NATURAL LEFT JOIN
  (SELECT * FROM my_table WHERE sellid IS NOT NULL) t2

See it on sqlfiddle.

But really, you should normalise your schema further so that you have a table of (personid, name) and a table of (personid, sellid) pairs (from which you essentially perform the above outer join as & when required to obtain the necessary records including NULLs).