MySQL :: MySQL 8.3 Reference Manual :: 5.3.4.8 Counting Rows

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5.3.4.8 行计数

数据库经常被用来回答问题：“某种类型的数据在表中出现了多少次？”例如，你可能想知道你有多少宠物，每个主人有多少宠物，或者你想对动物进行各种人口普查操作。

计算你拥有的总宠物数量与““宠物表中有多少行？””相同，因为每只宠物有一条记录。COUNT(*) 计数表中的行数，因此查询宠物数量的语句如下所示：

mysql> SELECT COUNT(*) FROM pet;
+----------+
| COUNT(*) |
+----------+
|        9 |
+----------+

早些时候，你检索了宠物主人的人名。你可以使用 COUNT() 来查找每个主人拥有的宠物数量：

mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner;
+--------+----------+
| owner  | COUNT(*) |
+--------+----------+
| Benny  |        2 |
| Diane  |        2 |
| Gwen   |        3 |
| Harold |        2 |
+--------+----------+

前面的查询使用 GROUP BY 将每个 owner 的所有记录分组。使用 COUNT() 与 GROUP BY 结合可以对数据进行各种分组分析。

每种物种的动物数量：

mysql> SELECT species, COUNT(*) FROM pet GROUP BY species;
+---------+----------+
| species | COUNT(*) |
+---------+----------+
| bird    |        2 |
| cat     |        2 |
| dog     |        3 |
| hamster |        1 |
| snake   |        1 |
+---------+----------+

每种性别的动物数量：

mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex;
+------+----------+
| sex  | COUNT(*) |
+------+----------+
| NULL |        1 |
| f    |        4 |
| m    |        4 |
+------+----------+

(在这个输出中，NULL 表示性别未知。)

每种物种和性别的动物数量：

mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | NULL |        1 |
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

你不需要检索整个表来使用 COUNT()。例如，前面的查询，只对狗和猫进行操作，语句如下所示：

mysql> SELECT species, sex, COUNT(*) FROM pet
       WHERE species = 'dog' OR species = 'cat'
       GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
+---------+------+----------+

或者，如果你想知道性别已知的动物数量：

mysql> SELECT species, sex, COUNT(*) FROM pet
       WHERE sex IS NOT NULL
       GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

如果你在选择的列中除了 COUNT() 值外，还选择了其他列，那么 GROUP BY 子句应该包含这些列。否则，将发生以下情况：

如果启用了 ONLY_FULL_GROUP_BY SQL 模式，将发生错误：

mysql> SET sql_mode = 'ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT owner, COUNT(*) FROM pet;
ERROR 1140 (42000): In aggregated query without GROUP BY, expression
#1 of SELECT list contains nonaggregated column 'menagerie.pet.owner';
this is incompatible with sql_mode=only_full_group_by

如果未启用 ONLY_FULL_GROUP_BY，查询将被处理为将所有行作为一个组，但选择的列值将是非确定性的。服务器可以从任何行中选择值：

mysql> SET sql_mode = '';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT owner, COUNT(*) FROM pet;
+--------+----------+
| owner  | COUNT(*) |
+--------+----------+
| Harold |        8 |
+--------+----------+
1 row in set (0.00 sec)

另见第 14.19.3 节，“MySQL Handling of GROUP BY”。见第 14.19.1 节，“Aggregate Function Descriptions” 了解 COUNT(expr) 行为和相关优化。

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