sql比较一个表中两个不同的组列的不同值

Question

Suppose one table t as following:

c1 c2
    == ==
    a  1
    a  2
    b  1
    b  2
    b  3
    c  4
    c  2

We group this table by c1 and has three group a, b, c. I need calculate the similarity of column c2 between two groups, like the following:

sim(a,b) = 2(common value of c2 are 1 and 2)/3(all value)=2/3
    sim(b,c) = 1(b and c has only one value 2 in common)/4 = 1/4
    sim(a,c) = 1/3

Can we use sql(Oracle 11g syntax first) to construct the expression above?

Answer 1

I believe this query does what you want:

select t1.c1, t2.c1, count(*) as NumInCommon,
       (select count(distinct t.c2)
        from t
        where t.c1 in (t1.c1, t2.c1)
       ) as NumInTotal
from t t1 join
     t t2
     on t1.c2 = t2.c2
group by t1.c1, t2.c1