Detailed explanation of examples of maximum and minimum values, sum query and count query in mysql-Mysql Tutorial-php.cn

Detailed explanation of examples of maximum and minimum values, sum query and count query in mysql

黄舟

Release： 2017-09-07 11:10:07

Original

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1. Maximum value query:

mysql> select max(score) from 4a;
+------------+
| max(score) |
+------------+
|         93 |
+------------+
1 row in set (0.06 sec)

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2. Minimum value query:

mysql> select max(4a.score),min(4inall.score) from 4a,4inall;
+---------------+-------------------+
| max(4a.score) | min(4inall.score) |
+---------------+-------------------+
|            93 |                35 |
+---------------+-------------------+
1 row in set (0.08 sec)

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3. Sum query and count query:

This is the original table, next The statements are all written based on this table.

mysql> select * from 4a;
+--------+------+--------+------+--------+------+------+-------+
| sname  | sage | tname  | t    | cname  | s    | c    | score |
+--------+------+--------+------+--------+------+------+-------+
| 刘一   |   18 | 叶平   |    1 | 语文   |    1 |    1 |    56 |
| 刘一   |   18 | 贺高   |    2 | 数学   |    1 |    2 |    78 |
| 刘一   |   18 | 杨艳   |    3 | 英语   |    1 |    3 |    67 |
| 刘一   |   18 | 周磊   |    4 | 物理   |    1 |    4 |    58 |
| 钱二   |   19 | 叶平   |    1 | 语文   |    2 |    1 |    79 |
| 钱二   |   19 | 贺高   |    2 | 数学   |    2 |    2 |    81 |
| 钱二   |   19 | 杨艳   |    3 | 英语   |    2 |    3 |    92 |
| 钱二   |   19 | 周磊   |    4 | 物理   |    2 |    4 |    68 |
| 张三   |   17 | 叶平   |    1 | 语文   |    3 |    1 |    91 |
| 张三   |   17 | 贺高   |    2 | 数学   |    3 |    2 |    47 |
| 张三   |   17 | 杨艳   |    3 | 英语   |    3 |    3 |    88 |
| 张三   |   17 | 周磊   |    4 | 物理   |    3 |    4 |    56 |
| 李四   |   18 | 贺高   |    2 | 数学   |    4 |    2 |    88 |
| 李四   |   18 | 杨艳   |    3 | 英语   |    4 |    3 |    90 |
| 李四   |   18 | 周磊   |    4 | 物理   |    4 |    4 |    93 |
| 王五   |   17 | 叶平   |    1 | 语文   |    5 |    1 |    46 |
| 王五   |   17 | 杨艳   |    3 | 英语   |    5 |    3 |    78 |
| 王五   |   17 | 周磊   |    4 | 物理   |    5 |    4 |    53 |
| 赵六   |   19 | 叶平   |    1 | 语文   |    6 |    1 |    35 |
| 赵六   |   19 | 贺高   |    2 | 数学   |    6 |    2 |    68 |
| 赵六   |   19 | 周磊   |    4 | 物理   |    6 |    4 |    71 |
| NULL   | NULL | NULL   | NULL | NULL   | NULL | NULL |    93 |
+--------+------+--------+------+--------+------+------+-------+
22 rows in set (0.00 sec)

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mysql> select sum(sname) from 4a;
+------------+
| sum(sname) |
+------------+
|          0 |
+------------+
1 row in set, 21 warnings (0.00 sec)

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We can see that although SNAME has many records, if you use SUM to query its sum, although there is no syntax error, the result is obviously "wrong".

mysql> select count(sname) from 4a;
+--------------+
| count(sname) |
+--------------+
|           21 |
+--------------+
1 row in set (0.00 sec)

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There is no problem in using count to count SNAME. Of course, NULL records will not be treated as a countable record.

mysql> select count(score) from 4a;
+--------------+
| count(score) |
+--------------+
|           22 |
+--------------+
1 row in set (0.00 sec)

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