A SELECT list cannot include both a group function, such as AVG, COUNT, MAX, MIN, SUM, STDDEV, or VARIANCE, and an individual column expression, unless the individual column expression is included in a GROUP BY clause.
Drop either the group function or the individual column expression from the SELECT list or add a GROUP BY clause that includes all individual column expressions listed.
eg:
SQL> select owner, count(*)
2 from dba_segments;
select owner, count(*)
*
ERROR at line 1:
ORA-00937: not a single-group group function
Because we have a group function (COUNT) in our column list, and an additional column (OWNER), we have to include the GROUP BY clause in our select statement.
SQL> select owner, count(*)
2 from dba_segments
3 group by owner;
OWNER COUNT(*)
------------------------------ ----------
USER1 166
USER2 166