Divide the columns in mysql into multiple columns - mysql

Question

I am completely new to mysql. Here I am trying to make a query in mysql which divides a column col1 into 4 different columns based on its category(col2) in sorted order as shown below. I have written a query like this till now:
select if(category = 'first',name ,NULL) as first,
if(category = 'second',name,NULL) as second,
if(category = 'third',name,NULL) as third,
if(category = 'fourth',name,NULL) as fourth
from 'table';
this code gives me the four columns but I am stuck now as I am not able to further filter this.
Given table:
name category
John first
Sunil third
Jenny third
Ashley fourth
Meera second
Abhay first
Required answer:
col1 col2 col3 col4
Abhay Meera Jenny Ashley
John NULL Sunil NULL
Note that all the columns in the answer are sorted.
Edit: I guess the question is not clear about the format of final answer. Thanks #philipxy for pointing out. Final answer should be adjusted in least number of rows (which is 2 in my case). All the columns should have equal number of rows and if some column has lesser values then that row will have NULL value in respective columns e.g col2 and col 4 above. Finally all the columns should be in sorted order where NULL will be in the last(if any)e.g Suppose there was an entry named Olly with category fourth then it should appear before NULL in col4 and after Ashley.

Answer 1

This is tricky. You are trying to stack lists vertically, rather than horizontally, which is not a "normal" SQL operation.
You can do what you want by using conditional aggregation. The problem is there is nothing to aggregate by. The solution is to introduce a variable column to calculate a sequence number for aggregation:
select max(case when category = 'first' then name end) as first,
max(case when category = 'second' then name end) as second,
max(case when category = 'third' then name end) as third,
max(case when category = 'fourth' then name end) as fourth
from (select t.*,
(#rn := if(#c = category, #rn + 1,
if(#c := category, 1, 1)
)
) as rn
from `table` t cross join
(select #c := '', #rn := 0) params
order by category
) t
group by rn;
If you want the values in a particular order in each column, then add a second sort key after category.
EDIT:
I should note that if you don't need multiple rows, but just the values, you can concatenate them together:
select group_concat(case when category = 'first' then name end) as firsts,
group_concat(case when category = 'second' then name end) as seconds,
group_concat(case when category = 'third' then name end) as thirds,
group_concat(case when category = 'fourth' then name end) as fourths
from`table` t
group by rn;