Mysql combinations of multiple coloumns - mysql

Question

I have my table in this format
item A B C D
i1 4 0 2 0
i2 0 2 1 0
i3 2 0 0 2
i4 3 0 1 1
And, I'm looking for output where two columns are taken in combinations and if both elements value is >0 output value is taken as 1. And total count is calculated from all records
w1 w2 out
A B 0
A C 2
A D 2
B C 1
B D 0
C D 1
i,e for columns (A,C)>0 only i1 and i4 satisfy.So out=2
So far, I have solved this by querying for each item and then summing the value in php. Can this be possible entirely by sql query?

Answer 1

You can do it in SQL, but I think you still have to consider all the different combinations. Here is one approach using union all and conditional aggregation:
select col1name, col2name,
sum(col1 > 0 and col2 > 0)
from (select 'A' as col1name, 'B' as col2name, A as col1, B as col2
from t
union all
select 'A', 'C', A, C
from t
union all
select 'A', 'D', A, D
from t
union all
select 'B', 'C', B, C
from t
union all
select 'B', 'D', B, D
from t
union all
select 'C', 'D', C, D
from t
) t
EDIT:
There is another way, if you unpivot the data. This starts with this query:
select item, n.colname,
(case when n.colname = 'A' then A
when n.colname = 'B' then B
when n.colname = 'C' then C
else D
end) as colval
from t cross join
(select ';A' as colname union all select 'B' union all select 'C' union all select 'D'
);
We can now do a self join on the query to get all combinations and aggregate to get the results:
select col1.colname, col2.colname,
sum(col1.colval > 0 and col2.colval > 0)
from (select item, n.colname,
(case when n.colname = 'A' then A
when n.colname = 'B' then B
when n.colname = 'C' then C
else D
end) as colval
from t cross join
(select ';A' as colname union all select 'B' union all select 'C' union all select 'D'
)
) col1 join
(select item, n.colname,
(case when n.colname = 'A' then A
when n.colname = 'B' then B
when n.colname = 'C' then C
else D
end) as colval
from t cross join
(select ';A' as colname union all select 'B' union all select 'C' union all select 'D'
)
) col2
on col1.item = col2.item and
col1.colname < col2.colname
group by col1.colname, col2.colname;
This version is simpler if you have more than four columns. The number of combinations in the first methods will quickly become cumbersome.