Oracle: Combine two group by queries (which use aggregate function count()) by union or so to get a consolidated result - oracle

Question

I have two tables. TABLE_A and TABLE_B.
Both tables maintain columns to save CREATION_USER. But this column has different name in respective tables.
My motive is to get a count of records each user has created in both tables.
That is, combining result of these two queries with few conditions. The user name should not get repeated and for user names who have created records in both tables, the count should be their sum.
SELECT A.CREATION_USER_A AS "USER",
COUNT(*)
FROM TABLE_A A
GROUP BY A.CREATION_USER_A;
SELECT B.CREATION_USER_B AS "USER",
COUNT(*)
FROM TABLE_B B
GROUP BY B.CREATION_USER_B;
For e.g.,
USER_A has created 2 records in TABLE_A,
USER_B has created 3 records in TABLE_B and
USER_C has created 4 records in TABLE_A and 3 records in TABLE_B.
So the output should look like this:
| USER | COUNT |
| USER_A | 2 |
| USER_B | 3 |
| USER_C | 7 |
I have written a query which does this but it performs really bad.
SELECT A.CREATION_USER_A AS "USER",
(COUNT(A.CREATION_USER_A)+(SELECT COUNT(CREATION_USER_B) FROM TABLE_B WHERE CREATION_USER_B = A.CREATION_USER_A)) AS "COUNT"
FROM TABLE_A A
GROUP BY A.CREATION_USER_A
UNION
SELECT B.CREATION_USER_B,
COUNT(B.CREATION_USER_B)
FROM TABLE_B B
WHERE B.CREATION_USER_B NOT IN (SELECT CREATION_USER_A FROM TABLE_A)
GROUP BY B.CREATION_USER_B;
Please suggest a way to get this done.

Answer 1

You can simply build a set given by the union (keeping duplicates) of all the records in your tables, and then count the records grouping by creation user:
Bulding some sample data:
create table table_a(id, creation_user_a) as (
select 1, 'USER_A' from dual union all
select 1, 'USER_A' from dual union all
select 1, 'USER_C' from dual union all
select 1, 'USER_C' from dual union all
select 1, 'USER_C' from dual union all
select 1, 'USER_C' from dual
);
create table table_b(id, creation_user_b) as (
select 1, 'USER_B' from dual union all
select 1, 'USER_B' from dual union all
select 1, 'USER_B' from dual union all
select 1, 'USER_C' from dual union all
select 1, 'USER_C' from dual union all
select 1, 'USER_C' from dual
)
The query:
select count(1), creation_user
from ( /* the union of all the records from table_a and table_b */
select creation_user_a as creation_user from table_a
union all /* UNION ALL keeps duplicates */
select creation_user_B from table_b
)
group by creation_user
order by creation_user
The result:
2 USER_A
3 USER_B
7 USER_C
The explain plan:
---------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 96 | 8 (25)| 00:00:01 |
| 1 | SORT ORDER BY | | 12 | 96 | 8 (25)| 00:00:01 |
| 2 | HASH GROUP BY | | 12 | 96 | 8 (25)| 00:00:01 |
| 3 | VIEW | | 12 | 96 | 6 (0)| 00:00:01 |
| 4 | UNION-ALL | | | | | |
| 5 | TABLE ACCESS FULL| TABLE_A | 6 | 48 | 3 (0)| 00:00:01 |
| 6 | TABLE ACCESS FULL| TABLE_B | 6 | 48 | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------

Answer 2

An alternative (but more complicated, and possibly slower - you'd need to test both to check) solution to Aleksej's answer is to use a full outer join to join both grouped by queries, like so:
WITH table_a AS (SELECT 'USER_A' creation_user_a, 10 val FROM dual UNION ALL
SELECT 'USER_A' creation_user_a, 20 val FROM dual UNION ALL
SELECT 'USER_C' creation_user_a, 30 val FROM dual UNION ALL
SELECT 'USER_C' creation_user_a, 40 val FROM dual UNION ALL
SELECT 'USER_C' creation_user_a, 50 val FROM dual UNION ALL
SELECT 'USER_C' creation_user_a, 60 val FROM dual),
table_b AS (SELECT 'USER_B' creation_user_b, 10 val FROM dual UNION ALL
SELECT 'USER_B' creation_user_b, 20 val FROM dual UNION ALL
SELECT 'USER_B' creation_user_b, 30 val FROM dual UNION ALL
SELECT 'USER_C' creation_user_b, 40 val FROM dual UNION ALL
SELECT 'USER_C' creation_user_b, 50 val FROM dual UNION ALL
SELECT 'USER_C' creation_user_b, 60 val FROM dual)
-- end of mimicking your tables with data in them. See the SQL below:
SELECT COALESCE(a.creation_user_a, b.creation_user_b) "USER",
nvl(a.cnt_a, 0) + nvl(b.cnt_b, 0) total_records
FROM (SELECT creation_user_a,
COUNT(*) cnt_a
FROM table_a
GROUP BY creation_user_a) a
FULL OUTER JOIN (SELECT creation_user_b,
COUNT(*) cnt_b
FROM table_b
GROUP BY creation_user_b) b ON a.creation_user_a = b.creation_user_b
ORDER BY "USER";
USER TOTAL_RECORDS
------ -------------
USER_A 2
USER_B 3
USER_C 7

Answer 3

Thank you for helping me guys. I have found a simpler and more efficient solution. It works.
SELECT CREATION_USER, SUM(TOTAL_COUNT) TOTAL_COUNT FROM
(SELECT /*+ PARALLEL */ A.CREATION_USER_A CREATION_USER,
COUNT(A.CREATION_USER_A) TOTAL_COUNT
FROM TABLE_A A
GROUP BY A.CREATION_USER_A
UNION
SELECT /*+ PARALLEL */ B.CREATION_USER_B CREATION_USER,
COUNT(B.CREATION_USER_B) TOTAL_COUNT
FROM TABLE_B B
GROUP BY B.CREATION_USER_B)
GROUP BY CREATION_USER;