Question

I’m having trouble with a query I’m trying to write in oracle.

Basically I want to get the count of two tables that have the same provider list. I can run the query individually but not sure how to do it in one shot.

Here’s a example:

SELECT name, COUNT(name)
FROM   table1
group by name

This gives me a list of names and a count beside them. If I run the same command but on table2(changing the from statement), it works fine. What I want to do is have a query that takes count from table1 and substracts it from count from table2.

How can I do it? I have tried to do nested selects and so on but can’t seem to get it to work.

Answer 1

One option assuming that the name values in both tables are identical would be

SELECT t1.name, t2.cnt - t1.cnt diff
  FROM( SELECT name, COUNT(name) cnt 
          FROM table1
         group by name ) t1,
      ( SELECT name, COUNT(name) cnt 
          FROM table2
         group by name ) t2
 WHERE t1.name = t2.name

If you want to handle the case where one table has a name that the other doesn’t but you don’t know which table has the extra name (and assuming that you want to say that the other table has a cnt of 0 for that name)

SELECT coalesce(t1.name, t2.name) name, 
       nvl(t1.cnt, 0 ) t1_cnt,
       nvl(t2.cnt, 0 ) t2_cnt,
       nvl(t2.cnt,0) - nvl(t1.cnt,0) diff
  FROM( SELECT name, COUNT(name) AS cnt
          FROM table1
         group by table1.name ) t1
      full outer join
      ( SELECT name, COUNT(name) AS cnt
          FROM table2
         group by table2.name ) t2
      on( t1.name = t2.name )

which you can see in this SQLFiddle