Question

Given the SQL code below:

create table A(a integer, b integer, c double);

insert into A(a, b, c)
values
(0, 0, 1.1),(1, 0, 1.2),(2, 0, 1.3),
(0, 1, 1.4),(1, 1, 1.5),(2, 1, 1.6),
(0, 2, 1.7),(1, 2, 1.8),(2, 2, 1.9),
(0, 0, 1.9),(1, 0, 1.8),(2, 0, 1.7),
(0, 1, 1.6),(1, 1, 1.5),(2, 1, 1.4),
(0, 2, 1.3),(1, 2, 1.2),(2, 2, 1.1)

mysql> select * from A where a = 0;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    0 |    0 |  1.1 |
|    0 |    1 |  1.4 |
|    0 |    2 |  1.7 |
|    0 |    0 |  1.9 |
|    0 |    1 |  1.6 |
|    0 |    2 |  1.3 |
+------+------+------+

mysql> select * from A where a = 0 group by b;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    0 |    0 |  1.1 |
|    0 |    1 |  1.4 |
|    0 |    2 |  1.7 |
+------+------+------+
3 rows in set (0.00 sec)

Why 3 rows? I would have thought both queries should return the same result since unique combinations of c exist.

Further

mysql> select *, sum(c) from A group by b;
+------+------+------+--------+
| a    | b    | c    | sum(c) |
+------+------+------+--------+
|    0 |    0 |  1.1 |      9 |
|    0 |    1 |  1.4 |      9 |
|    0 |    2 |  1.7 |      9 |
+------+------+------+--------+
3 rows in set (0.00 sec)

the sum seems to be the sum of all the rows rather than the sum of the groupings

Any ideas why I’m seeing this strange behaviour?

Answer 1

Here is clearly mentioned

In standard SQL, a query that includes a GROUP BY clause cannot refer
to nonaggregated columns in the select list that are not named in the
GROUP BY clause. For example, this query is illegal in standard SQL
because the name column in the select list does not appear in the
GROUP BY:

SELECT o.custid, c.name, MAX(o.payment)
  FROM orders AS o, customers AS c
  WHERE o.custid = c.custid
  GROUP BY o.custid;

For the query to be legal, the name column must be omitted from the
select list or named in the GROUP BY clause.

MySQL extends the use of GROUP BY so that the select list can refer to
nonaggregated columns not named in the GROUP BY clause.

References:

http://rpbouman.blogspot.com/2007/05/debunking-group-by-myths.html

http://explainextended.com/2011/03/30/mysql-group-by-in-union/

http://20bits.com/articles/10-tips-for-optimizing-mysql-queries-that-dont-suck/