Question

table

 id | name | year

Data:

 1  | ham   | 2006
 2  | ham   | 2007
 3  | ham   | 2008
 4  | amm   | 2007
 5  | amm   | 2008
 6  | inn   | 2009

Now i’m trying to construct a sql that gives me the following output (or similar)

 name  | y1   | y2   | y3   | y4    
 ham   | 2006 | 2007 | 2008 | null
 amm   | null | 2007 | 2008 | null
 inn   | null | null | null | 2009

When doing multiple (self) left joins I kind of gets this, but only if 2006 is set for that name. Is there a way to achieve this?

Answer 1

You want what is called a “pivot”, and call you need is a conditional sum:

select
   name,
   sum(year = 2006) as y1,
   sum(year = 2007) as y2,
   sum(year = 2008) as y3,
   sum(year = 2009) as y4
from mytable
group by name;

No joins, no muss, no fuss. And it will perform really well.

This works because in mysql, true is 1 and false is 0, so summing a condition counts how many times it was true!

Note that this will give you zeroes instead of nulls for “no data” years, which is probably better. If you really want nulls, use if(sum(year = 2006) = 0, null, sum(year = 2006)) as y1 etc, but hopefully you don’t need it.