子查询
/*
含义：
出现在其他语句中的select语句，称为子查询或内查询
外部的查询语句，称为主查询或外查询
分类：
按子查询出现的位置：
select后面：
仅仅支持标量子查询
from后面：
支持表子查询
where或having后面：★
标量子查询（单行） √
列子查询 （多行） √
行子查询
exists后面（相关子查询）
表子查询
按结果集的行列数不同：
标量子查询（结果集只有一行一列）
列子查询（结果集只有一列多行）
行子查询（结果集有一行多列）
表子查询（结果集一般为多行多列）
*/
#一、where或having后面
/*
1、标量子查询（单行子查询）
2、列子查询（多行子查询）
3、行子查询（多列）
特点：
①子查询放在小括号内
②子查询一般放在条件的右侧
③标量子查询，一般搭配着单行操作符使用
> < >= <= = <>
列子查询，一般搭配着多行操作符使用
in/not in 等于列表中的任意一个、
any/some 和子查询返回的某一个值比较、min替换
all和子查询返回的所有比较 max替换
④子查询的执行优先于主查询执行，主查询的条件用到了子查询的结果
*/
#1.标量子查询★
#案例1：谁的工资比 Abel 高?
#①查询Abel的工资
SELECT salary FROM employees WHERE last_name = ‘Abel‘
#②查询员工的信息，满足 salary>①结果
SELECT * FROM employees WHERE salary>( SELECT salary FROM employees WHERE last_name = ‘Abel‘ );
#案例2：返回job_id与141号员工相同，salary比143号员工多的员工 姓名，job_id 和工资
#①查询141号员工的job_id
SELECT job_id FROM employees WHERE employee_id = 141
#②查询143号员工的salary
SELECT salary FROM employees WHERE employee_id = 143
#③查询员工的姓名，job_id 和工资，要求job_id=①并且salary>②
SELECT last_name,job_id,salary FROM employees WHERE job_id = ( SELECT job_id FROM employees WHERE employee_id = 141 ) AND salary>( SELECT salary FROM employees WHERE employee_id = 143 );
#案例3：返回公司工资最少的员工的last_name,job_id和salary
#①查询公司的 最低工资 SELECT MIN(salary) FROM employees #②查询last_name,job_id和salary，要求salary=① SELECT last_name,job_id,salary FROM employees WHERE salary=( SELECT MIN(salary) FROM employees );
#案例4：查询最低工资大于50号部门最低工资的部门id和其最低工资
#①查询50号部门的最低工资 SELECT MIN(salary) FROM employees WHERE department_id = 50 #②查询每个部门的最低工资 SELECT MIN(salary),department_id FROM employees GROUP BY department_id #③ 在②基础上筛选，满足min(salary)>① SELECT MIN(salary),department_id FROM employees GROUP BY department_id HAVING MIN(salary)>( SELECT MIN(salary) FROM employees WHERE department_id = 50 );
#非法使用标量子查询
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
SELECT salary
FROM employees
WHERE department_id = 250
);
#2.列子查询（多行子查询）★
#案例1：返回location_id是1400或1700的部门中的所有员工姓名
#①查询location_id是1400或1700的部门编号 SELECT DISTINCT department_id FROM departments WHERE location_id IN(1400,1700) #②查询员工姓名，要求部门号是①列表中的某一个 SELECT last_name FROM employees WHERE department_id <>ALL( SELECT DISTINCT department_id FROM departments WHERE location_id IN(1400,1700) );
#案例2：返回其它工种中比job_id为‘IT_PROG’工种任一工资低的员工的员工号、姓名、job_id 以及salary
#①查询job_id为‘IT_PROG’部门任一工资 SELECT DISTINCT salary FROM employees WHERE job_id = ‘IT_PROG‘ #②查询员工号、姓名、job_id 以及salary，salary<(①)的任意一个 SELECT last_name,employee_id,job_id,salary FROM employees WHERE salary<ANY( SELECT DISTINCT salary FROM employees WHERE job_id = ‘IT_PROG‘ ) AND job_id<>‘IT_PROG‘; #或 SELECT last_name,employee_id,job_id,salary FROM employees WHERE salary<( SELECT MAX(salary) FROM employees WHERE job_id = ‘IT_PROG‘ ) AND job_id<>‘IT_PROG‘;
#案例3：返回其它部门中比job_id为‘IT_PROG’部门所有工资都低的员工 的员工号、姓名、job_id 以及salary
SELECT last_name,employee_id,job_id,salary FROM employees WHERE salary<ALL( SELECT DISTINCT salary FROM employees WHERE job_id = ‘IT_PROG‘ ) AND job_id<>‘IT_PROG‘;
#或
SELECT last_name,employee_id,job_id,salary FROM employees WHERE salary<( SELECT MIN( salary) FROM employees WHERE job_id = ‘IT_PROG‘ ) AND job_id<>‘IT_PROG‘;
#3、行子查询（结果集一行多列或多行多列）
#案例：查询员工编号最小并且工资最高的员工信息
SELECT * FROM employees WHERE (employee_id,salary)=( SELECT MIN(employee_id),MAX(salary) FROM employees ); #①查询最小的员工编号 SELECT MIN(employee_id) FROM employees #②查询最高工资 SELECT MAX(salary) FROM employees #③查询员工信息 SELECT * FROM employees WHERE employee_id=( SELECT MIN(employee_id) FROM employees )AND salary=( SELECT MAX(salary) FROM employees );
#二、select后面
/*
仅仅支持标量子查询
*/
#案例：查询每个部门的员工个数
SELECT d.*,( SELECT COUNT(*) FROM employees e WHERE e.department_id = d.`department_id` ) 个数 FROM departments d;
#案例2：查询员工号=102的部门名 只能一行一列
SELECT ( SELECT department_name FROM departments d INNER JOIN employees e ON d.department_id=e.department_id WHERE e.employee_id=102 ) 部门名;
#三、from后面
/*
将子查询结果充当一张表，要求必须起别名
*/
#案例：查询每个部门的平均工资的工资等级
#①查询每个部门的平均工资 SELECT AVG(salary),department_id FROM employees GROUP BY department_id SELECT * FROM job_grades; #②连接①的结果集和job_grades表，筛选条件平均工资 between lowest_sal and highest_sal SELECT ag_dep.*,g.`grade_level` FROM ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ) ag_dep INNER JOIN job_grades g ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;
#四、exists后面（相关子查询）
/*
语法：
exists(完整的查询语句)
结果：
1或0
*/
SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);
#案例1：查询有员工的部门名
#in SELECT department_name FROM departments d WHERE d.`department_id` IN( SELECT department_id FROM employees ) #exists SELECT department_name FROM departments d WHERE EXISTS( SELECT * FROM employees e WHERE d.`department_id`=e.`department_id` );
#案例2：查询没有女朋友的男神信息
#in SELECT bo.* FROM boys bo WHERE bo.id NOT IN( SELECT boyfriend_id FROM beauty ) #exists SELECT bo.* FROM boys bo WHERE NOT EXISTS( SELECT boyfriend_id FROM beauty b WHERE bo.`id`=b.`boyfriend_id` );
#1. 查询和Zlotkey相同部门的员工姓名和工资
#①查询Zlotkey的部门 SELECT department_id FROM employees WHERE last_name = ‘Zlotkey‘ #②查询部门号=①的姓名和工资 SELECT last_name,salary FROM employees WHERE department_id = ( SELECT department_id FROM employees WHERE last_name = ‘Zlotkey‘ )
#2.查询工资比公司平均工资高的员工的员工号，姓名和工资。
#①查询平均工资 SELECT AVG(salary) FROM employees #②查询工资>①的员工号，姓名和工资。 SELECT last_name,employee_id,salary FROM employees WHERE salary>( SELECT AVG(salary) FROM employees );
#3.查询各部门中工资比本部门平均工资高的员工的员工号, 姓名和工资
#①查询各部门的平均工资 SELECT AVG(salary),department_id FROM employees GROUP BY department_id #②连接①结果集和employees表，进行筛选 SELECT employee_id,last_name,salary,e.department_id FROM employees e INNER JOIN ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ) ag_dep ON e.department_id = ag_dep.department_id WHERE salary>ag_dep.ag ;
#4. 查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名
#①查询姓名中包含字母u的员工的部门 SELECT DISTINCT department_id FROM employees WHERE last_name LIKE ‘%u%‘ #②查询部门号=①中的任意一个的员工号和姓名 SELECT last_name,employee_id FROM employees WHERE department_id IN( SELECT DISTINCT department_id FROM employees WHERE last_name LIKE ‘%u%‘ );
#5. 查询在部门的location_id为1700的部门工作的员工的员工号
#①查询location_id为1700的部门 SELECT DISTINCT department_id FROM departments WHERE location_id = 1700 #②查询部门号=①中的任意一个的员工号 SELECT employee_id FROM employees WHERE department_id =ANY( SELECT DISTINCT department_id FROM departments WHERE location_id = 1700 );
#6.查询管理者是King的员工姓名和工资
#①查询姓名为king的员工编号 SELECT employee_id FROM employees WHERE last_name = ‘K_ing‘ #②查询哪个员工的manager_id = ① SELECT last_name,salary FROM employees WHERE manager_id IN( SELECT employee_id FROM employees WHERE last_name = ‘K_ing‘ );
#7.查询工资最高的员工的姓名，要求first_name和last_name显示为一列，列名为 姓.名
#①查询最高工资 SELECT MAX(salary) FROM employees #②查询工资=①的姓.名 SELECT CONCAT(first_name,last_name) "姓.名" FROM employees WHERE salary=( SELECT MAX(salary) FROM employees );
分页查询 ★
/*
应用场景：当要显示的数据，一页显示不全，需要分页提交sql请求
语法：
select 查询列表 7
from 表 1
【join type join 表2 2
on 连接条件 3
where 筛选条件 4
group by 分组字段 5
having 分组后的筛选 6
order by 排序的字段】 8
limit 【offset,】size; 9
offset要显示条目的起始索引（起始索引从0开始）
size 要显示的条目个数
特点：
①limit语句放在查询语句的最后
②公式
要显示的页数 page，每页的条目数size
select 查询列表
from 表
limit (page-1)*size,size;
size=10
page
1 0
2 10
3 20
*/
#案例1：查询前五条员工信息
SELECT * FROM employees LIMIT 0,5; SELECT * FROM employees LIMIT 5;
#案例2：查询第11条——第25条
SELECT * FROM employees LIMIT 10,15;
#案例3：有奖金的员工信息，并且工资较高的前10名显示出来
SELECT * FROM employees WHERE commission_pct IS NOT NULL ORDER BY salary DESC LIMIT 10
联合查询
/*
union 联合 合并：将多条查询语句的结果合并成一个结果
语法：
查询语句1
union
查询语句2
union
...
应用场景：
要查询的结果来自于多个表，且多个表没有直接的连接关系，但查询的信息一致时
特点：★
1、要求多条查询语句的查询列数是一致的！
2、要求多条查询语句的查询的每一列的类型和顺序最好一致
3、union关键字默认去重，如果使用union all 可以包含重复项
*/
#引入的案例：查询部门编号>90或邮箱包含a的员工信息
SELECT * FROM employees WHERE email LIKE ‘%a%‘ OR department_id>90; SELECT * FROM employees WHERE email LIKE ‘%a%‘ UNION SELECT * FROM employees WHERE department_id>90;
#案例：查询中国用户中男性的信息以及外国用户中年男性的用户信息
SELECT id,cname FROM t_ca WHERE csex=‘男‘ UNION ALL SELECT t_id,tname FROM t_ua WHERE tGender=‘male‘;
# 1. 查询工资最低的员工信息: last_name, salary
#①查询最低的工资 SELECT MIN(salary) FROM employees #②查询last_name,salary，要求salary=① SELECT last_name,salary FROM employees WHERE salary=( SELECT MIN(salary) FROM employees );
# 2. 查询平均工资最低的部门信息
#方式一： #①各部门的平均工资 SELECT AVG(salary),department_id FROM employees GROUP BY department_id #②查询①结果上的最低平均工资 SELECT MIN(ag) FROM ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ) ag_dep #③查询哪个部门的平均工资=② SELECT AVG(salary),department_id FROM employees GROUP BY department_id HAVING AVG(salary)=( SELECT MIN(ag) FROM ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ) ag_dep ); #④查询部门信息 SELECT d.* FROM departments d WHERE d.`department_id`=( SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary)=( SELECT MIN(ag) FROM ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ) ag_dep ) );
#方式二：
#①各部门的平均工资 SELECT AVG(salary),department_id FROM employees GROUP BY department_id #②求出最低平均工资的部门编号 SELECT department_id FROM employees GROUP BY department_id ORDER BY AVG(salary) LIMIT 1; #③查询部门信息 SELECT * FROM departments WHERE department_id=( SELECT department_id FROM employees GROUP BY department_id ORDER BY AVG(salary) LIMIT 1 ); # 3.
查询平均工资最低的部门信息和该部门的平均工资
#①各部门的平均工资 SELECT AVG(salary),department_id FROM employees GROUP BY department_id #②求出最低平均工资的部门编号 SELECT AVG(salary),department_id FROM employees GROUP BY department_id ORDER BY AVG(salary) LIMIT 1; #③查询部门信息 SELECT d.*,ag FROM departments d JOIN ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ORDER BY AVG(salary) LIMIT 1 ) ag_dep ON d.`department_id`=ag_dep.department_id;
# 4. 查询平均工资最高的 job 信息
#①查询最高的job的平均工资 SELECT AVG(salary),job_id FROM employees GROUP BY job_id ORDER BY AVG(salary) DESC LIMIT 1 #②查询job信息 SELECT * FROM jobs WHERE job_id=( SELECT job_id FROM employees GROUP BY job_id ORDER BY AVG(salary) DESC LIMIT 1 );
# 5. 查询平均工资高于公司平均工资的部门有哪些?
#①查询平均工资 SELECT AVG(salary) FROM employees #②查询每个部门的平均工资 SELECT AVG(salary),department_id FROM employees GROUP BY department_id #③筛选②结果集，满足平均工资>① SELECT AVG(salary),department_id FROM employees GROUP BY department_id HAVING AVG(salary)>( SELECT AVG(salary) FROM employees );
# 6. 查询出公司中所有 manager 的详细信息.
#①查询所有manager的员工编号 SELECT DISTINCT manager_id FROM employees #②查询详细信息，满足employee_id=① SELECT * FROM employees WHERE employee_id =ANY( SELECT DISTINCT manager_id FROM employees );
# 7. 各个部门中 最高工资中最低的那个部门的 最低工资是多少
#①查询各部门的最高工资中最低的部门编号 SELECT department_id FROM employees GROUP BY department_id ORDER BY MAX(salary) LIMIT 1 #②查询①结果的那个部门的最低工资 SELECT MIN(salary) ,department_id FROM employees WHERE department_id=( SELECT department_id FROM employees GROUP BY department_id ORDER BY MAX(salary) LIMIT 1 );
# 8. 查询平均工资最高的部门的 manager 的详细信息: last_name, department_id, email, salary
#①查询平均工资最高的部门编号 SELECT department_id FROM employees GROUP BY department_id ORDER BY AVG(salary) DESC LIMIT 1 #②将employees和departments连接查询，筛选条件是① SELECT last_name, d.department_id, email, salary FROM employees e INNER JOIN departments d ON d.manager_id = e.employee_id WHERE d.department_id = (SELECT department_id FROM employees GROUP BY department_id ORDER BY AVG(salary) DESC LIMIT 1) ;
#一、查询每个专业的学生人数
SELECT majorid,COUNT(*) FROM student GROUP BY majorid;
#二、查询参加考试的学生中，每个学生的平均分、最高分
SELECT AVG(score),MAX(score),studentno FROM result GROUP BY studentno;
#三、查询姓张的每个学生的最低分大于60的学号、姓名
SELECT s.studentno,s.`studentname`,MIN(score) FROM student s JOIN result r ON s.`studentno`=r.`studentno` WHERE s.`studentname` LIKE ‘张%‘ GROUP BY s.`studentno` HAVING MIN(score)>60; #四、查询每个专业生日在“1988-1-1”后的学生姓名、专业名称 SELECT m.`majorname`,s.`studentname` FROM student s JOIN major m ON m.`majorid`=s.`majorid` WHERE DATEDIFF(borndate,‘1988-1-1‘)>0 GROUP BY m.`majorid`;
#五、查询每个专业的男生人数和女生人数分别是多少
SELECT COUNT(*),sex,majorid FROM student GROUP BY sex,majorid;
#六、查询专业和张翠山一样的学生的最低分
#①查询张翠山的专业编号 SELECT majorid FROM student WHERE studentname = ‘张翠山‘ #②查询编号=①的所有学生编号 SELECT studentno FROM student WHERE majorid=( SELECT majorid FROM student WHERE studentname = ‘张翠山‘ ) #②查询最低分 SELECT MIN(score) FROM result WHERE studentno IN( SELECT studentno FROM student WHERE majorid=( SELECT majorid FROM student WHERE studentname = ‘张翠山‘ ) )
#七、查询大于60分的学生的姓名、密码、专业名
SELECT studentname,loginpwd,majorname FROM student s JOIN major m ON s.majorid= m.majorid JOIN result r ON s.studentno=r.studentno WHERE r.score>60;
#八、按邮箱位数分组，查询每组的学生个数
SELECT COUNT(*),LENGTH(email) FROM student GROUP BY LENGTH(email); #九、查询学生名、专业名、分数 SELECT studentname,score,majorname FROM student s JOIN major m ON s.majorid= m.majorid LEFT JOIN result r ON s.studentno=r.studentno
#十、查询哪个专业没有学生，分别用左连接和右连接实现
#左 SELECT m.`majorid`,m.`majorname`,s.`studentno` FROM major m LEFT JOIN student s ON m.`majorid` = s.`majorid` WHERE s.`studentno` IS NULL; #右 SELECT m.`majorid`,m.`majorname`,s.`studentno` FROM student s RIGHT JOIN major m ON m.`majorid` = s.`majorid` WHERE s.`studentno` IS NULL;
#十一、查询没有成绩的学生人数
SELECT COUNT(*) FROM student s LEFT JOIN result r ON s.`studentno` = r.`studentno` WHERE r.`id` IS NULL
SELECT 查询字段 7
FROM 表 1
JOIN 连接类型 2
ON 连接条件 3
WHERE 筛选条件 4
GROUP by分组列表 5
HAVING 分组后的筛选 6
ORDER BY 排序列表 8
LIMIT 偏移 条目数 9