在 SQL 中，没有 Count(*) 或 Sum()、Max()、avg()、...，Group By 是什么意思，它有什么用途？

Question

在 SQL 中，如果我们使用 Group By 而不使用 Count(*) 或 Sum() 等，那么结果如下：

mysql>  select * from sentGifts;
+--------+------------+--------+------+---------------------+--------+
| sentID | whenSent   | fromID | toID | trytryWhen          | giftID |
+--------+------------+--------+------+---------------------+--------+
|      1 | 2010-04-24 |    123 |  456 | 2010-04-24 01:52:20 |    100 |
|      2 | 2010-04-24 |    123 | 4568 | 2010-04-24 01:56:04 |    100 |
|      3 | 2010-04-24 |    123 | NULL | NULL                |      1 |
|      4 | 2010-04-24 |   NULL |  111 | 2010-04-24 03:10:42 |      2 |
|      5 | 2010-03-03 |     11 |   22 | 2010-03-03 00:00:00 |      6 |
|      6 | 2010-04-24 |     11 |  222 | 2010-04-24 03:54:49 |      6 |
|      7 | 2010-04-24 |      1 |    2 | 2010-04-24 03:58:45 |      6 |
+--------+------------+--------+------+---------------------+--------+
7 rows in set (0.00 sec)


mysql>  select *, count(*) from sentGifts group by whenSent;
+--------+------------+--------+------+---------------------+--------+----------+
| sentID | whenSent   | fromID | toID | trytryWhen          | giftID | count(*) |
+--------+------------+--------+------+---------------------+--------+----------+
|      5 | 2010-03-03 |     11 |   22 | 2010-03-03 00:00:00 |      6 |        1 |
|      1 | 2010-04-24 |    123 |  456 | 2010-04-24 01:52:20 |    100 |        6 |
+--------+------------+--------+------+---------------------+--------+----------+
2 rows in set (0.00 sec)


mysql>  select * from sentGifts group by whenSent;
+--------+------------+--------+------+---------------------+--------+
| sentID | whenSent   | fromID | toID | trytryWhen          | giftID |
+--------+------------+--------+------+---------------------+--------+
|      5 | 2010-03-03 |     11 |   22 | 2010-03-03 00:00:00 |      6 |
|      1 | 2010-04-24 |    123 |  456 | 2010-04-24 01:52:20 |    100 |
+--------+------------+--------+------+---------------------+--------+
2 rows in set (0.00 sec)

每个“组”仅返回 1 行。当使用“Group By”时没有“Count(*)”等是什么意思，它有什么用？谢谢。

Answer 1

默认情况下，MySQL 将返回执行查询时遇到的第一行的值。就像它使用默认的arbitrary.

如果您有一个很长的列列表，并且您知道其中大部分都是重复的，那么这非常有用，例如：

Login    LongName             City        PhoneNr      Time
Dude     Mr. Dude the 2nd     Jerk Town   12345678     13:01
Dude     Mr. Dude the 2nd     Jerk Town   12345678     13:05
Dude     Mr. Dude the 2nd     Jerk Town   12345678     13:12

在这里你可以group by login：

select LongName, City, PhoneNr, max(Time) from Logins group by login

因为您知道这Long Name取决于Login，所以这将按预期工作。我所知道的所有其他 DBMS 系统都要求您显式指定group by login, LongName, City, PhoneNr。即使在 MySQL 中，这也被认为是很好的实践。