Question

Execute below script for create test table.

create table if not exists t1 (id1 int,id2 int);

Now, table is created, and it is empty table.

Execute below script,

select max(id1), max(id2) from t1

It will return below result(one row).

max(id1)      max(id2)
-----------   --------
<null>        <null>

Execute below script,

select max(id1), max(id2) from t1 group by id1,id2

It will return below result(no result).

max(id1)      max(id2)
-----------   --------

Is there somebody explain the reason?

Answer 1

The documentation says:

The max() aggregate function returns the maximum value of all values in the group. […] Aggregate max() returns NULL if and only if there are no non-NULL values in the group.

In your first query, there is one group which does not have any records.

In your second query, there is no group.