网站seo啥意思怎么做,360浏览器直接进入网站,安徽安搜做的网站怎么样,海港区网站快排seo文章目录 什么是子查询需求分析与问题解决子查询的基本语法结构子查询的分类 单行子查询单行比较操作符代码示例HAVING 中的子查询CASE中的子查询子查询中的空值问题非法使用子查询 多行子查询多行比较操作符代码示例空值问题 相关子查询代码示例在ORDER BY 中使用子查询EXISTS… 文章目录 什么是子查询需求分析与问题解决子查询的基本语法结构子查询的分类 单行子查询单行比较操作符代码示例HAVING 中的子查询CASE中的子查询子查询中的空值问题非法使用子查询 多行子查询多行比较操作符代码示例空值问题 相关子查询代码示例在ORDER BY 中使用子查询EXISTS 与 NOT EXISTS关键字 相关更新相关删除 思考题练习 什么是子查询
子查询指一个查询语句嵌套在另一个查询语句内部的查询这个特性从MySQL 4.1开始引入。
SQL 中子查询的使用大大增强了 SELECT 查询的能力因为很多时候查询需要从结果集中获取数据或者需要从同一个表中先计算得出一个数据结果然后与这个数据结果可能是某个标量也可能是某个集合进行比较。
需求分析与问题解决
查询员工中谁的工资比Abel工资高
这种问题我们得先知道Abel的工资
#方式一
SELECT salary
FROM employees
WHERE last_name Abel;
SELECT last_name,salary
FROM employees
WHERE salary 11000;
#方式二自连接
SELECT t1.salary
FROM employees t1 JOIN employees t2
WHERE t2.last_name Abel AND t1.salary t2.salary;子查询
select salary
from employees
where salary (SELECT salaryFROM employeesWHERE last_name Abel)子查询的基本语法结构 子查询内查询在主查询之前一次执行完成。子查询的结果被主查询外查询使用 。
注意事项
子查询要包含在括号内将子查询放在比较条件的右侧单行操作符对应单行子查询多行操作符对应多行子查询
子查询的分类 我们按内查询的结果返回一条还是多条记录将子查询分为 单行子查询 、 多行子查询 。 我们按内查询是否被执行多次将子查询划分为 相关(或关联)子查询 和 不相关(或非关联)子查询 。 子查询从数据表中查询了数据结果如果这个数据结果只执行一次然后这个数据结果作为主查询的条件进行执行那么这样的子查询叫做不相关子查询。 同样如果子查询需要执行多次即采用循环的方式先从外部查询开始每次都传入子查询进行查询然后再将结果反馈给外部这种嵌套的执行方式就称为相关子查询。
单行子查询
单行比较操作符
操作符含义equal togreater thangreater than or equal toless thanless than or equal tonot eq
单行子查询也就是内部查询的结果是唯一的
代码示例
题目查询工资大于149号员工工资的员工的信息
SELECT *
FROM employees
WHERE salary (SELECT salaryFROM employeesWHERE employee_id 149)题目返回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
SELECT job_id,salary
FROM employees
WHERE salary (SELECT salaryFROM employeesWHERE employee_id 143)
AND job_id (SELECT job_idFROM employeesWHERE employee_id 141)题目返回公司工资最少的员工的last_name,job_id和salary
SELECT last_name,job_id,salary
FROM employees
WHERE salary ( SELECT MIN(salary)FROM employees)题目查询与141号或174号员工的manager_id和department_id相同的其他员工的employee_idmanager_iddepartment_id
SELECT employee_id, manager_id, department_id
FROM employees
WHERE manager_id (SELECT manager_idFROM employeesWHERE employee_id 141)
AND department_id (SELECT department_idFROM employeesWHERE employee_id 141)
AND employee_id 141#成对比较
SELECT employee_id, manager_id, department_id
FROM employees
WHERE (manager_id, department_id) (SELECT manager_id, department_idFROM employeesWHERE employee_id 141)
AND employee_id 141;HAVING 中的子查询
首先执行子查询。向主查询中的HAVING 子句返回结果。
题目查询最低工资大于50号部门最低工资的部门id和其最低工资
SELECT department_id,MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary) (SELECT MIN(salary)FROM employeesWHERE department_id 50)CASE中的子查询
题目显式员工的employee_id,last_name和location。其中若员工department_id与location_id为1800的department_id相同则location为’Canada’其余则为’USA’
SELECT employee_id,last_name,(CASE department_idWHEN (SELECT department_id FROM departments WHERE location_id 1800) THEN CanadaELSE USAEND)location
FROM employees子查询中的空值问题
#空值问题
SELECT last_name, job_id
FROM employees
WHERE job_id ( SELECT job_idFROM employeesWHERE last_name Haas);子查询不返回任何行 非法使用子查询
SELECT employee_id, last_name
FROM employees
WHERE salary ( SELECT MIN(salary)FROM employeesGROUP BY department_id);多行子查询使用单行比较符 多行子查询
也称为集合比较子查询内查询返回多行使用多行比较操作符
多行比较操作符
操作符含义IN等于列表中的任意一个ANY需要和单行比较操作符一起使用和子查询返回的某一个值比较ALL需要和单行比较操作符一起使用和子查询返回的所有值比较SOME实际上是ANY的别名作用相同一般常使用ANY
代码示例
题目返回其它job_id中比job_id为‘IT_PROG’部门任一工资低的员工的员工号、姓名、job_id 以及salary SELECT employee_id,last_name,job_id,salaryFROM employeesWHERE salary ANY(SELECT salaryFROM employeesWHERE job_id IT_PROG)
AND job_id IT_PROG题目返回其它job_id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 employee_id,last_name,job_id,salaryFROM employeesWHERE salary all(SELECT salaryFROM employeesWHERE job_id IT_PROG)
AND job_id IT_PROG题目查询平均工资最低的部门id
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) ALL (SELECT AVG(salary)FROM employeesGROUP BY department_id)
#方式2
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) (SELECT MIN(avg_sal)FROM (SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id) dept_avg_sal)空值问题
SELECT last_name
FROM employees
WHERE employee_id NOT IN (SELECT manager_idFROM employees);相关子查询
如果子查询的执行依赖于外部查询通常情况下都是因为子查询中的表用到了外部的表并进行了条件关联因此每执行一次外部查询子查询都要重新计算一次这样的子查询就称之为 关联子查询 。相关子查询按照一行接一行的顺序执行主查询的每一行都执行一次子查询。 代码示例
题目查询员工中工资大于本部门平均工资的员工的last_name,salary和其department_id
#方式一相关子查询
SELECT t1.last_name,t1.salary,t1.department_id
FROM employees t1
WHERE salary (SELECT mIN(salary)FROM employees t2WHERE t2.department_id t1.department_id)
#方式二在 FROM 中使用子查询
SELECT last_name,salary,e1.department_id
FROM employees e1,(SELECT department_id,AVG(salary) dept_avg_sal FROM employees GROUP BY department_id) e2
WHERE e1.department_id e2.department_id
AND e2.dept_avg_sal e1.salary;题目若employees表中employee_id与job_history表中employee_id相同的数目不小于2输出这些相同id的员工的employee_id,last_name和其job_id
SELECT employee_id,last_name,job_id
FROM employees t1
WHERE 2 ( SELECT COUNT(*)FROM job_history t2WHERE t1.employee_id t2.employee_id)在ORDER BY 中使用子查询
题目查询员工的id,salary,按照department_name 排序
SELECT employee_id,salary
FROM employees t1
ORDER BY ( SELECT department_name FROM departments t2WHERE t2.department_id t1.department_id) ASCEXISTS 与 NOT EXISTS关键字
关联子查询通常也会和 EXISTS操作符一起来使用用来检查在子查询中是否存在满足条件的行。
如果在子查询中不存在满足条件的行
条件返回 FALSE继续在子查询中查找
如果在子查询中存在满足条件的行
不在子查询中继续查找条件返回 TRUE
NOT EXISTS关键字表示如果不存在某种条件则返回TRUE否则返回FALSE。
题目查询公司管理者的employee_idlast_namejob_iddepartment_id信息
#使用非关联子查询
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees)
#自连接
SELECT DISTINCT e1.employee_id, e1.last_name, e1.job_id, e1.department_id
FROM employees e1 JOIN employees e2
WHERE e1.employee_id e2.manager_id;
#使用EXISTS操作符
SELECT employee_id,last_name,job_id,department_id
FROM employees t1
WHERE EXISTS (SELECT AFROM employees t2WHERE t1.employee_id t2.manager_id )第一种写法in执行过程中只有两次查询先进行内部查询然后根据内部查询的结果筛选外部条件子查询会缓存到内存中适合于子查询结果较小的情况效率比较高但是需要使用内存空间第三种exists比较耗时每次都是从外部查询中取出记录和内部查询匹配内部的查询不会产生临时表不耗费内存空间适合于内部查询较大且内存放不下的情况
题目查询departments表中不存在于employees表中的部门的department_id和department_name
SELECT department_id,department_name
FROM departments d1
WHERE NOT EXISTS (SELECT AFROM employees t1WHERE d1.department_id t1.department_id)相关更新
UPDATE table1 alias1
SET column (SELECT expressionFROM table2 alias2WHERE alias1.column alias2.column);使用相关子查询依据一个表中的数据更新另一个表的数据。 题目在employees中增加一个department_name字段数据为员工对应的部门名称
ALTER TABLE employees
ADD(department_name VARCHAR(14));UPDATE employees e1
SET department_name ( SELECT department_nameFROM departments d1WHERE e1.department_id d1.department_id )相关删除
DELETE FROM table1 alias1
WHERE column operator (SELECT expression
FROM table2 alias2
WHERE alias1.column alias2.column);题目删除表employees中其与emp_history表皆有的数据
DELETE FROM employees e
WHERE employee_id in( SELECT employee_idFROM emp_historyWHERE employee_id e.employee_id);思考题
问题谁的工资比Abel的高
#方式1自连接
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e1.last_name Abel
AND e1.salary e2.salary#方式2子查询
SELECT last_name,salary
FROM employees
WHERE salary (SELECT salaryFROM employeesWHERE last_name Abel);问题以上两种方式有好坏之分吗 解答自连接方式好 题目中可以使用子查询也可以使用自连接。一般情况建议你使用自连接因为在许多 DBMS 的处理过程中对于自连接的处理速度要比子查询快得多。 可以这样理解子查询实际上是通过未知表进行查询后的条件判断而自连接是通过已知的自身数据表进行条件判断因此在大部分 DBMS 中都对自连接处理进行了优化。
练习
#1.查询和Zlotkey相同部门的员工姓名和工资
SELECT last_name,salary
FROM employees
WHERE department_id (SELECT department_idFROM employeesWHERE 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。
SELECT last_name,salary,employee_id
from employees
where salary (select avg(salary)from employees)
#3.选择工资大于所有JOB_ID SA_MAN的员工的工资的员工的last_name, job_id, salary
SELECT last_name, job_id, salary
FROM employees
WHERE salary ALL (SELECT salaryFROM employeesWHERE JOB_ID SA_MAN)
#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
SELECT last_name,employee_id
FROM employees
WHERE department_id IN (SELECT department_idFROM employeesWHERE last_name LIKE %u%)
SELECT last_name,employee_id
FROM employees
WHERE department_id any (SELECT department_idFROM employeesWHERE 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
SELECT employee_id
FROM employees
WHERE department_id IN (SELECT department_idFROM departmentsWHERE location_id 1700
)
#6.查询管理者是King的员工姓名和工资
SELECT last_name,salary
FROM employees
WHERE manager_id ANY (SELECT employee_idFROM employeesWHERE last_name King)
#7.查询工资最低的员工信息: last_name, salary
SELECT last_name,salary
FROM employees
WHERE salary (SELECT MIN(salary)FROM employees)
#8.查询平均工资最低的部门信息
SELECT *
FROM departments
WHERE department_id (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) ALL (SELECT AVG(salary)FROM employeesGROUP BY department_id))
SELECT *
FROM departments
WHERE department_id (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) (SELECT MIN(dept_avgsal)FROM (SELECT AVG(salary) dept_avgsalFROM employeesGROUP BY department_id) avg_sal));
SELECT *
FROM departments
WHERE department_id (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) (SELECT AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_salLIMIT 0,1))
SELECT d.*
FROM departments d,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_salLIMIT 0,1) dept_avg_sal
WHERE d.department_id dept_avg_sal.department_id
#9.查询平均工资最低的部门信息和该部门的平均工资相关子查询
SELECT d.*, (SELECT AVG(salary) FROM employees WHERE department_id d.department_id ) minavg
FROM departments AS d
HAVING d.department_id (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) ALL (SELECT AVG(salary)FROM employeesGROUP BY department_id ))
SELECT d.*,dept_avg_sal.avg_sal
FROM departments d,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP BY department_idORDER BY avg_salLIMIT 0,1) dept_avg_sal
WHERE d.department_id dept_avg_sal.department_id
#10.查询平均工资最高的 job 信息
SELECT *
FROM jobs
WHERE job_id (SELECT job_idFROM employeesGROUP BY job_idHAVING AVG(salary) ALL(SELECT AVG(salary)FROM employeesGROUP BY job_id))
SELECT j.*
FROM jobs j,(SELECT job_id,AVG(salary) avg_salFROM employeesGROUP BY job_idORDER BY avg_sal DESCLIMIT 0,1 ) job_avg_sal
WHERE j.job_id job_avg_sal.job_id
#11.查询平均工资高于公司平均工资的部门有哪些?
SELECT *
FROM departments
WHERE department_id IS NOT NULL
AND department_id IN (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) (SELECT AVG(salary)FROM employees))
SELECT department_id
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING AVG(salary) (SELECT AVG(salary)FROM employees);
#12.查询出公司中所有 manager 的详细信息
SELECT *
FROM employees
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employeesWHERE manager_id IS NOT NULL)
#自连接
SELECT DISTINCT e1.employee_id, e1.last_name, e1.salary
FROM employees e1 JOIN employees e2
WHERE e1.employee_id e2.manager_id;
#exist
SELECT *
FROM employees e
WHERE EXISTS (SELECT AFROM employeesWHERE manager_id e.employee_id)#13.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?
select min(salary)
from employees
where department_id (select department_idfrom employeesgroup by department_idorder by Max(salary)limit 0,1)
SELECT MIN(salary)
FROM employees
WHERE department_id (SELECT department_idFROM employeesGROUP BY department_idHAVING MAX(salary) (SELECT MIN(max_sal)FROM (SELECT MAX(salary) max_salFROM employeesGROUP BY department_id) dept_max_sal));
SELECT employee_id,MIN(salary)
FROM employees e,(SELECT department_id,MAX(salary) max_salFROM employeesGROUP BY department_idORDER BY max_salLIMIT 0,1) dept_max_sal
WHERE e.department_id dept_max_sal.department_id
#14.查询平均工资最高的部门的 manager 的详细信息: last_name, department_id, email, salary
SELECT *
FROM employees
WHERE employee_id (SELECT manager_idFROM departmentsWHERE department_id (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) ALL (SELECT AVG(salary)FROM employeesGROUP BY department_id)))
#15. 查询部门的部门号其中不包括job_id是ST_CLERK的部门号
SELECT department_id
FROM departments d
WHERE department_id NOT IN (SELECT DISTINCT department_idFROM employeesWHERE job_id ST_CLERK);
SELECT department_id
FROM departments d
WHERE NOT EXISTS (SELECT * FROM employees eWHERE d.department_id e.department_idAND job_id ST_CLERK)
#16. 选择所有没有管理者的员工的last_name
select last_name
from employees
where employee_id in (select employee_idfrom employeeswhere manager_id is null)
SELECT last_name
FROM employees e1
WHERE NOT EXISTS (SELECT *FROM employees e2WHERE e1.manager_id e2.employee_id
);
#17查询员工号、姓名、雇用时间、工资其中员工的管理者为 De Haan
SELECT employee_id, last_name, hire_date, salary
FROM employees
WHERE manager_id (SELECT employee_idFROM employeesWHERE last_name De Haan
)
SELECT employee_id, last_name, hire_date, salary
FROM employees e1
WHERE EXISTS (SELECT * FROM employees e2WHERE e1.manager_id e2.employee_idAND e2.last_name De Haan)
#18.查询各部门中工资比本部门平均工资高的员工的员工号, 姓名和工资相关子查询
SELECT e1.employee_id, e1.last_name, e1.salary
FROM employees e1
WHERE salary (SELECT AVG(salary)FROM employees WHERE manager_id e1.manager_id)
#方式二
SELECT employee_id,last_name,salary
FROM employees e1,( SELECT department_id,AVG(salary) avg_salFROM employees e2 GROUP BY department_id) dept_avg_sal
WHERE e1.department_id dept_avg_sal.department_id
AND e1.salary dept_avg_sal.avg_sal;
#19.查询每个部门下的部门人数大于 5 的部门名称相关子查询
SELECT department_name,department_id
FROM departments d
WHERE 5 (SELECT COUNT(*)FROM employees eWHERE d.department_id e.department_id);
#20.查询每个国家下的部门个数大于 2 的国家编号相关子查询
SELECT country_id
FROM locations l
WHERE 2 (SELECT COUNT(*)FROM departments dWHERE l.location_id d.location_id
);
