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ORA-00979: not a GROUP BY expression

A select with a group by clause requires all non-aggregated columns to be also present in the group by clause, otherwise, Oracle throws a ORA-00979: not a GROUP BY expression error message, as demonstrated below:

create table tq84_00979 (
    val    number,
    grp_1  varchar2(10),
    grp_2  varchar2(10),
    col    date
);

--
--  Cause ORA-00979
-- (column grp_2 is selected but
--  not part of the group by
--  expression)
--
select
   max(val),
   grp_1,
   grp_2
from
   tq84_00979
group by
   grp_1;

--
-- This one is OK
--
select
   max(val),
   grp_1,
   grp_2
from
   tq84_00979
group by
   grp_1,
   grp_2;

--
-- This one also causes an ORA-00979 because
-- of the col expression in the order by clause
--
select
   max(val),
   grp_1,
   grp_2
from
   tq84_00979
group by
   grp_1,
   grp_2
order by
   col;

drop table tq84_00979;

Improved error messages in 23c

The ORA-00979 error message belongs to the error messages that were improved in the 23c release: in this release, it now lists the (first?) expression missing in the group by clause.

ORA-00979: not a GROUP BY expression

Improved error messages in 23c

See also