Sql Query: co-occurrence of column values

Question

I have a table like this:

col1     col2
id1      item1
id1      item2
id1      item3
id2      item1
id2      item4
id3      item2
id3      item3

I have to run a query on this table to find the number of times each pair of items shared a common id. For ex, in the above case, the pair (item1, item2) has a count of 1 (only id1 has both item1 and item2). Similarly, the pair (item2, item3) has a count of 2 (id1, id3).

I can write code to achieve this, but I am not able to come with a sql query. Help me write an efficient query to output the following:

col1    col2    count   
item1   item2    1
item1   item3    1
item1   item4    1
item2   item3    2 

Thanks

Answer 1

select    t1.col2  as item_A
         ,t2.col2  as item_B
         ,count(*) as cnt

from                mytable t1 
          join      mytable t2
          on        t1.col1 = t2.col1

where     t1.col2 < t2.col2

group by  t1.col2
         ,t2.col2

---

+--------+--------+-----+
| item_a | item_b | cnt |
+--------+--------+-----+
| item1  | item2  |   1 |
| item1  | item3  |   1 |
| item1  | item4  |   1 |
| item2  | item3  |   2 |
+--------+--------+-----+