- 问题
练习常见的SQL查询及条件设置
- 创建stu_info表，并插入数据
- 练习常见SQL查询及条件设置
- 步骤
实现此案例需要按照如下步骤进行。
步骤一：根据任务要求建立员工档案表stu_info
1）在test库中创建stu_info表
以root用户登入MySQL服务器：
[root@dbsvr1 ~]# mysql -u root -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 5
Server version: 5.6.15 MySQL Community Server (GPL)
Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql>
打开test库：
mysql> USE test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
创建stu_info表，包括name、gender、age三个字段：
mysql> CREATE TABLE stu_info(
-> name varchar(12) NOT NULL,
-> gender enum('boy','girl') DEFAULT 'boy',
-> age int(3) NOT NULL
-> );
Query OK, 0 rows affected (0.03 sec)
确认表结构：
mysql> DESC stu_info;
+--------+--------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+--------------------+------+-----+---------+-------+
| name | varchar(12) | NO | | NULL | |
| gender | enum('boy','girl') | YES | | boy | |
| age | int(3) | NO | | NULL | |
+--------+--------------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
2）准备测试表格
向建立的stu_info表插入几条测试记录
mysql> INSERT INTO stu_info VALUES
-> ('Jim','girl',24),
-> ('Tom','boy',21),
-> ('Lily','girl',20),
-> ('Jerry','boy',27),
-> ('Mike','boy',21)
-> ;
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
确认stu_info表的所有记录内容：
mysql> SELECT * FROM stu_info;
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jim | girl | 24 |
| Tom | boy | 21 |
| Lily | girl | 20 |
| Jerry | boy | 27 |
| Mike | boy | 21 |
+-------+--------+-----+
5 rows in set (0.00 sec)
步骤二：练习常见SQL查询及条件设置
1）常用的表记录统计函数
查询stu_info表一共有多少条记录（本例中为5条）：
mysql> SELECT count(*) FROM stu_info;
+----------+
| count(*) |
+----------+
| 5 |
+----------+
1 row in set (0.00 sec)
计算stu_info表中各学员的平均年龄、最大年龄、最小年龄：
mysql> SELECT avg(age),max(age),min(age) FROM stu_info;
+----------+----------+----------+
| avg(age) | max(age) | min(age) |
+----------+----------+----------+
| 22.6000 | 27 | 20 |
+----------+----------+----------+
1 row in set (0.03 sec)
计算stu_info表中男学员的个数：
mysql> SELECT count(gender) FROM stu_info WHERE gender='boy';
+---------------+
| count(gender) |
+---------------+
| 3 |
+---------------+
1 row in set (0.00 sec)
2）字段值的数值比较
列出stu_info表中年龄为21岁的学员记录：
mysql> SELECT * FROM stu_info WHERE age=21;
+------+--------+-----+
| name | gender | age |
+------+--------+-----+
| Tom | boy | 21 |
| Mike | boy | 21 |
+------+--------+-----+
2 rows in set (0.00 sec)
列出stu_info表中年龄超过21岁的学员记录：
mysql> SELECT * FROM stu_info WHERE age>21;
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jim | girl | 24 |
| Jerry | boy | 27 |
+-------+--------+-----+
2 rows in set (0.00 sec)
列出stu_info表中年龄大于或等于21岁的学员记录：
mysql> SELECT * FROM stu_info WHERE age>=21;
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jim | girl | 24 |
| Tom | boy | 21 |
| Jerry | boy | 27 |
| Mike | boy | 21 |
+-------+--------+-----+
4 rows in set (0.00 sec)
列出stu_info表中年龄在20岁和24岁之间的学员记录：
mysql> SELECT * FROM stu_info WHERE age BETWEEN 20 and 24;
+------+--------+-----+
| name | gender | age |
+------+--------+-----+
| Jim | girl | 24 |
| Tom | boy | 21 |
| Lily | girl | 20 |
| Mike | boy | 21 |
+------+--------+-----+
4 rows in set (0.00 sec)
3）多个条件的组合
列出stu_info表中年龄小于23岁的女学员记录：
mysql> SELECT * FROM stu_info WHERE age < 23 AND gender='girl';
+------+--------+-----+
| name | gender | age |
+------+--------+-----+
| Lily | girl | 20 |
+------+--------+-----+
1 row in set (0.00 sec)
列出stu_info表中年龄小于23岁的学员，或者女学员的记录：
mysql> SELECT * FROM stu_info WHERE age < 23 OR gender='girl';
+------+--------+-----+
| name | gender | age |
+------+--------+-----+
| Jim | girl | 24 |
| Tom | boy | 21 |
| Lily | girl | 20 |
| Mike | boy | 21 |
+------+--------+-----+
4 rows in set (0.00 sec)
如果某个记录的姓名属于指定范围内的一个，则将其列出：
mysql> SELECT * FROM stu_info WHERE name IN
-> ('Jim','Tom','Mickey','Minnie');
+------+--------+-----+
| name | gender | age |
+------+--------+-----+
| Jim | girl | 24 |
| Tom | boy | 21 |
+------+--------+-----+
2 rows in set (0.00 sec)
4）使用SELECT做数学计算
计算1234与5678的和：
mysql> SELECT 1234+5678;
+-----------+
| 1234+5678 |
+-----------+
| 6912 |
+-----------+
1 row in set (0.00 sec)
计算1234与5678的乘积：
mysql> SELECT 1234*5678;
+-----------+
| 1234*5678 |
+-----------+
| 7006652 |
+-----------+
1 row in set (0.00 sec)
计算1.23456789除以3的结果：
mysql> SELECT 1.23456789/3;
+----------------+
| 1.23456789/3 |
+----------------+
| 0.411522630000 |
+----------------+
1 row in set (0.00 sec)
输出stu_info表各学员的姓名、15年后的年龄：
mysql> SELECT name,age+15 FROM stu_info;
+-------+--------+
| name | age+15 |
+-------+--------+
| Jim | 39 |
| Tom | 36 |
| Lily | 35 |
| Jerry | 42 |
| Mike | 36 |
+-------+--------+
5 rows in set (0.00 sec)
5）使用模糊查询，LIKE引领
以下划线 _ 匹配单个字符，% 可匹配任意多个字符。
列出stu_info表中姓名以“J”开头的学员记录：
mysql> SELECT * FROM stu_info WHERE name LIKE 'J%';
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jim | girl | 24 |
| Jerry | boy | 27 |
+-------+--------+-----+
2 rows in set (0.00 sec)
列出stu_info表中姓名以“J”开头且只有3个字母的学员记录：
mysql> SELECT * FROM stu_info WHERE name LIKE 'J__';
+------+--------+-----+
| name | gender | age |
+------+--------+-----+
| Jim | girl | 24 |
+------+--------+-----+
1 row in set (0.00 sec)
6）使用正则表达式，REGEXP引领
列出stu_info表中姓名以“J”开头且以“y”结尾的学员记录：
mysql> SELECT * FROM stu_info WHERE name REGEXP '^J.*y$';
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jerry | boy | 27 |
+-------+--------+-----+
1 row in set (0.00 sec)
效果等同于：
mysql> SELECT * FROM stu_info WHERE name Like 'J%y';
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jerry | boy | 27 |
+-------+--------+-----+
1 row in set (0.00 sec)
列出stu_info表中姓名以“J”开头或者以“y”结尾的学员记录：
mysql> SELECT * FROM stu_info WHERE name REGEXP '^J|y$';
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jim | girl | 24 |
| Lily | girl | 20 |
| Jerry | boy | 27 |
+-------+--------+-----+
3 rows in set (0.00 sec)
效果等同于：
mysql> SELECT * FROM stu_info WHERE name Like 'J%' OR name Like '%y';
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jim | girl | 24 |
| Lily | girl | 20 |
| Jerry | boy | 27 |
+-------+--------+-----+
3 rows in set (0.00 sec)
7）按指定的字段排序，ORDER BY
列出stu_info表的所有记录，按年龄排序：
mysql> SELECT * FROM stu_info GROUP BY age;
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Lily | girl | 20 |
| Tom | boy | 21 |
| Jim | girl | 24 |
| Jerry | boy | 27 |
+-------+--------+-----+
4 rows in set (0.00 sec)
因默认为升序（Ascend）排列，所以上述操作等效于：
mysql> SELECT * FROM stu_info GROUP BY age ASC;
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Lily | girl | 20 |
| Tom | boy | 21 |
| Jim | girl | 24 |
| Jerry | boy | 27 |
+-------+--------+-----+
4 rows in set (0.00 sec)
若要按降序（Descend）排列，则将ASC改为DESC即可：
mysql> SELECT * FROM stu_info GROUP BY age DESC;
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jerry | boy | 27 |
| Jim | girl | 24 |
| Tom | boy | 21 |
| Lily | girl | 20 |
+-------+--------+-----+
4 rows in set (0.00 sec)
8）限制查询结果的输出条数，LIMIT
查询stu_info表的所有记录，只列出前3条：
mysql> SELECT * FROM stu_info LIMIT 3;
+------+--------+-----+
| name | gender | age |
+------+--------+-----+
| Jim | girl | 24 |
| Tom | boy | 21 |
| Lily | girl | 20 |
+------+--------+-----+
3 rows in set (0.00 sec)
列出stu_info表中年龄最大的3条学员记录：
mysql> SELECT * FROM stu_info GROUP BY age DESC LIMIT 3;
+-------+--------+-----+
| name | gender | age |
+-------+--------+-----+
| Jerry | boy | 27 |
| Jim | girl | 24 |
| Tom | boy | 21 |
+-------+--------+-----+
3 rows in set (0.00 sec)
9）分组查询结果，GROUP BY
针对stu_info表，按性别分组，分别统计出男、女学员的人数：
mysql> SELECT gender,count(gender) FROM stu_info GROUP BY gender;
+--------+---------------+
| gender | count(gender) |
+--------+---------------+
| boy | 3 |
| girl | 2 |
+--------+---------------+
2 rows in set (0.00 sec)
列出查询字段时，可以通过AS关键字来指定显示别名，比如上述操作可改为：
mysql> SELECT gender AS '性别',count(gender) AS '人数'
-> FROM stu_info GROUP BY gender;
+--------+--------+
| 性别 | 人数 |
+--------+--------+
| boy | 3 |
| girl | 2 |
+--------+--------+
2 rows in set (0.00 sec)