mysql - SUM(a*b) 有什么作用？

Question

我以前从未见过 SUM(a*b)。我希望有人能告诉我它的预期行为，以及它是 SQL 标准的一部分，还是 MySQL 恰好实现的东西。一个例子：

localhost test> CREATE TABLE numbers (id int not null primary key auto_increment, a int, b int);
Query OK, 0 rows affected (0.03 sec)

localhost test> INSERT INTO numbers (a,b) VALUES (1, 2), (3,4),(1,1),(5,6);
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

localhost test> select * FROM numbers GROUP BY id;
+----+------+------+
| id | a    | b    |
+----+------+------+
|  1 |    1 |    2 |
|  2 |    3 |    4 |
|  3 |    1 |    1 |
|  4 |    5 |    6 |
+----+------+------+
4 rows in set (0.00 sec)

localhost test> select SUM(a), SUM(b) FROM numbers GROUP BY id;
+--------+--------+
| SUM(a) | SUM(b) |
+--------+--------+
|      1 |      2 |
|      3 |      4 |
|      1 |      1 |
|      5 |      6 |
+--------+--------+
4 rows in set (0.00 sec)

localhost test> select SUM(a*b), SUM(a), SUM(b) FROM numbers GROUP BY id;
+----------+--------+--------+
| SUM(a*b) | SUM(a) | SUM(b) |
+----------+--------+--------+
|        2 |      1 |      2 |
|       12 |      3 |      4 |
|        1 |      1 |      1 |
|       30 |      5 |      6 |
+----------+--------+--------+
4 rows in set (0.00 sec)

localhost test> alter table numbers add col2 int not null;
Query OK, 4 rows affected (0.03 sec)
Records: 4  Duplicates: 0  Warnings: 0

localhost test> update numbers set col2 = id %2;
Query OK, 2 rows affected (0.00 sec)
Rows matched: 4  Changed: 2  Warnings: 0

localhost test> select col2, SUM(a*b), SUM(a), SUM(b) FROM numbers GROUP BY col2;
+------+----------+--------+--------+
| col2 | SUM(a*b) | SUM(a) | SUM(b) |
+------+----------+--------+--------+
|    0 |       42 |      8 |     10 |
|    1 |        3 |      2 |      3 |
+------+----------+--------+--------+
2 rows in set (0.01 sec)

我预计最后一行有 80 和 30，或者如果它使用 MySQL 选择一行的非标准行为（我启用了 SQL_MODE=ONLY_FULL_GROUP_BY）会产生错误。

Answer 1

该表达式SUM(a*b)是完全有效的标准 SQL。它计算a * b每一行，并对每个组内的结果求和（如果未指定 GROUP BY，则为整个表）。

+----+------+------+------+------+
| 编号 | col2 | 一个 | 乙 | a*b |
+----+------+------+------+------+
| 1 | 1 | 1 | 2 | 2 |                  
| 2 | 0 | 3 | 4 | 12 | --+
| 3 | 1 | 1 | 1 | 1 | |---> 12 + 30 = 42
| 4 | 0 | 5 | 6 | 30 | --+
+----+------+------+------+-----|

它不一样SUM(a) * SUM(b)。