【Mysql】三、Mysql基础篇 --- 查询
查询篇
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- 子查询
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- 一、WHERE 子查询
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- 1、单行子查询
- 2、列子查询
- 3、行子查询
- 二、SELECT 子查询
- 三、FROM 子查询
- 四、EXISTS 子查询
- 子查询案例分析
- 分页查询
- 联合查询
- 行转列、列转行
子查询
一、WHERE 子查询
1、单行子查询
案例1：谁的工资比w.deng高
-- 1、查询w.deng的工资
SELECT salary FROM users WHERE username='w.deng'
-- 2、查询工资大于w.deng的salary
SELECT * FROM users
WHERE salary > (SELECT salary FROM users WHERE username='w.deng')
案例2：查询和141号员工同部门（dep_id）并且工资比143号工资高的所有员工
-- 1、查询141号id的员工的dep_id
SELECT dep_id FROM users WHERE id=141
-- 2、查询143号id的员工的salary
SELECT salary FROM users WHERE id=143
-- 3、查询语句
SELECT * FROM users
WHERE dep_id=(SELECT dep_id FROM users WHERE id=141)
AND salary>(SELECT salary FROM users WHERE id=143)
案例3：查询工资最少的员工
SELECT * FROM users WHERE salary=(SELECT MIN(salary) FROM users)
案例4：查询比部门10最低薪资高的所有员工
SELECT * FROM users
WHERE salary > (SELECT MIN(salary) FROM users where dep_id=10)
2、列子查询
案例1：loc_id为1400和1700下所有部门员工姓名
SELECT * FROM users
WHERE dep_id IN (SELECT dep_id FROM users WHERE loc_id IN (1400,1700))
3、行子查询
用处不多,不需要了解
SELECT * FROM users WHERE (dep_id,id)=(SELECT MAX(dep_id), MIN(id) FROM users)
二、SELECT 子查询
select后面的子查询支持单行单列
SELECT (
SELECT porjects.name FROM users
LEFT JOIN projects ON projects.user_id = users.id
WHERE users.id=102
) 102用户负责项目名称
三、FROM 子查询
使用子查询 每个部门平均薪资的薪资等级
-- salary_grade 员工等级表
SELECT avg_dep.*, salary_grade.grade
FROM (
SELECT AVG(salary) AS avg_sal, dep_id
FROM users
GROUP BY dep_id
) avg_dep
INNER JOIN salary_grade ON avg_dep.avg_sal BETWEEN salary_grade.lower_sal AND salary_grade.high_sal
四、EXISTS 子查询
查询存在结果集就返回1，反之不存在返回0
存在结果集
SELECT EXISTS(SELECT * FROM users)
不存在结果集
SELECT EXISTS(SELECT * FROM users WHERE username='22222')
子查询案例分析
1、查询各部门比本部门平均工资高的员工
-- 1、查询各部门的平均工资
SELECT AVG(salary) avg_salary, dep_id
FROM users
GROUP BY dep_id
-- 2、查询各部门比本平均工资高的员工
SELECT * FROM users
INNER JOIN (
SELECT AVG(salary) avg_salary, dep_id
FROM users
GROUP BY dep_id
) avg_salary_dep
ON avg_salary_dep.dep_id = users.dep_id AND users.salary > avg_salary_dep.avg_salary
2、查询ring管理者的所有员工姓名
SELECT * FROM users
WHERE margin_id IN (SELECT id FROM users WHERE username='ring')
3、查询工资最低的员工姓名和工资
-- 方法1：
SELECT * FROM (SELECT MIN(salary) sal FROM users) user_min_salary
INNER JOIN users ON user_min_salary.sal = users.salary
-- 方法2：
SELECT * FROM users
WHERE salary = (SELECT MIN(salary) sal FROM users)
4、查询平均工资最低的部门
-- 方法1：
SELECT *
FROM (SELECT AVG(salary) avg_sal, dep_id FROM users GROUP BY dep_id) dep_avg
INNER JOIN dep ON dep.id = dep_avg.dep_id
ORDER BY avg_sal ASC LIMIT 1
-- 方法2：
SELECT * FROM dep
WHERE id=(
SELECT dep_id FROM users
GROUP BY dep_id
ORDER BY AVG(salary)
LIMIT 1
)
5、查询各部门的最高工资
-- 查询各部门的最高工资
SELECT * FROM dep
INNER JOIN (
SELECT MAX(salary), dep_id FROM users
GROUP BY dep_id
) max_sal ON max_sal.dep_id = dep.id
6、那个部门最低工资>5000
-- 那个部门最低工资>5000
SELECT MIN(salary) min_sal, dep_id FROM users
GROUP BY dep_id
HAVING min_sal > 5000
分页查询
关键词：limit
案例1：查询users表前两条数据
-- SELECT * FROM users LIMIT offset,size offset：启始条目位置（非id） size：显示条目个数
SELECT * FROM users LIMIT 0,2
--如果起始位置是第一条，可省略offset
SELECT * FROM users LIMIT 2
查询结果：
案例2：分页公式
(page-1)*size,size
SELECT * FROM users LIMIT (page-1)*size,size
联合查询
UNION ALL 联合查询 将多条语句合并成一个结果集
查询规则：
- 多条查询列数是一致的
- 字段名默认第一条语句的字段名称
- 多条语句查询列顺序保持一致
UNION默认去重，UNION ALL显示所有结果，包含重复的项
案例1：普通查询和联合查询
-- 普通查询
SELECT * FROM users WHERE username LIKE "zhang%" OR salary > 1500
-- 两种查询 结果集相等
-- 联合查询
SELECT * FROM users WHERE username LIKE "zhang%"
UNION
SELECT * FROM users WHERE salary > 1500
行转列、列转行
- 数据准备
CREATE TABLE students (
id INT NOT NULL auto_increment PRIMARY KEY,
name VARCHAR(20),
subject VARCHAR(20),
score int
)
INSERT INTO students (name, subject, score) VALUES ('小明', '语文', 80),('小明', '英语', 40),('小明', '数学', 90),('阿花', '语文', 99),('阿花', '英语', 90),('阿花', '数学', 10)
- 行转列
-- 创建视图，行转列做准备
-- CREATE VIEW max_source
SELECT
name,
MAX(
CASE subject
WHEN '语文'
THEN score
ELSE 0
END
) AS '语文',
MAX(
CASE subject
WHEN '英语'
THEN score
ELSE 0
END
) AS '英语',
MAX(
CASE subject
WHEN '数学'
THEN score
ELSE 0
END
) AS '数学'
FROM students
GROUP BY name
- 列转行
SELECT name, '语文' AS subject, MAX(`语文`) AS score FROM max_source
GROUP BY name
UNION ALL
SELECT name, '数学' AS subject, MAX(`数学`) AS score FROM max_source
GROUP BY `name`
UNION ALL
SELECT name, '英语' AS subject, MAX(`英语`) AS score FROM max_source
GROUP BY `name`
- 扩展联系
每科最高分是多少
-- 方法1:mysql传统方法子查询 SELECT students.name, students.subject, students.score FROM ( SELECT subject, MAX(score) AS ms FROM students GROUP BY subject ) max_sorce LEFT JOIN students ON students.subject = max_sorce.subject AND max_sorce.ms = students.score -- 方法2:mysql8.0新特性，OVER() SELECT subject, name, score, RANK() OVER(PARTITION BY subject ORDER BY score DESC) AS rank_num FROM students