#如下是一个查询语句
-- 使用的是select ... from
/*
这是多行注释Java语言的分类：
1. 基本语法
2. oop
3. io \ 多线程 \ 网络编程 \ 反射 \ 集合 等SQL的分类：DML : INSERT \ DELETE \ UPDATE \ SELECT
DDL : CREATE \ ALTER \ DROP \ RENAME \ TRUNCATE
DCL : COMMIT \ ROLLBACK \ GRANT \ REVOKE \ SAVEPOINT*/
SELECT *
FROM employees

#02-基本的SELECT语句USE temp; #使用指定的数据库#1. 基本使用
SELECT employee_id,last_name,EMAIL
FROM employees;SELECT department_id,manager_id,location_id
FROM departments;SELECT * # * : 代表所有的字段
FROM employees;SELECT *
FROM countries;#2. 列的别名
#as : alias
#可以使用一对""，给列起别名
SELECT employee_id emp_id,last_name AS lname,salary "monthly salary"
FROM employees;#3. 去除重复行
SELECT DISTINCT department_id
FROM employees;#如下操作是错误的
SELECT employee_id, DISTINCT department_id
FROM employees;#4. 空值问题
#空值，表示没有赋值，理解为null。
#空值参与运算的问题：结果也为空。
#空值，不等同于0,'','null'
SELECT employee_id,commission_pct,salary,salary * (1 + commission_pct),
salary * (1 + IFNULL(commission_pct,0)) "real_salary"
FROM employees;#5. 显示表结构
DESC employees;DESCRIBE employees;

#03-过滤数据
#查询90号部门员工的信息SELECT employee_id,last_name,department_id,salary
FROM employees
# 使用where实现数据的过滤
# where 必须紧跟在from子句的后面
WHERE department_id = 90;SELECT employee_id,last_name,department_id,salary
FROM employees
#where salary > 5000;
WHERE department_id <> 90;# 2. between 下边界 and 上边界 (包含边界)
#查询工资大于等于6000 且小于等于 8000之间的员工信息
SELECT employee_id,salary
FROM employees
WHERE salary >= 6000 AND salary <= 8000;
#where salary between 6000 and 8000;
#WHERE salary BETWEEN 8000 AND 6000;#3. in(set):
SELECT employee_id,department_id,salary
FROM employees
#where department_id = 30 or department_id = 40 or department_id = 50;
#where department_id in (30,40,50);
WHERE salary IN (6000,7000,8000);#4. like ： 模糊查询#精确查询
SELECT employee_id,LAST_NAME
FROM employees
WHERE last_name = 'King';  #字符串、日期需要使用一对''表示。注意，不要使用""
#where hire_date = '1993-01-13'; #查询姓名中包含字符'a'的员工
# % : 表示0个，1个或多个字符
SELECT employee_id,LAST_NAME
FROM employees
WHERE last_name LIKE '%a%';# 查询姓名中包含字符'a'且包含字符'e'的员工SELECT employee_id,LAST_NAME
FROM employees
#WHERE last_name LIKE '%a%e%' or last_name LIKE '%e%a%';
WHERE last_name LIKE '%a%' AND last_name LIKE '%e%';# 查询姓名中第3个字符是a的员工
# _ : 表示一个不确定的字符
SELECT employee_id,LAST_NAME
FROM employees
WHERE last_name LIKE '__a%';# 查询姓名中第2个字符是_且第3个字符是a的员工
SELECT employee_id,LAST_NAME
FROM employees
#WHERE last_name LIKE '_\_a%';  #  sout("林辉很\"帅\"");
WHERE last_name LIKE '_#_a%' ESCAPE '#';# 5. is null:空值SELECT last_name,commission_pct
FROM employees
WHERE commission_pct IS NULL;SELECT last_name,commission_pct
FROM employees
#WHERE !(commission_pct IS NULL);
WHERE commission_pct IS NOT NULL;# 6. 算术运算符
SELECT employee_id,salary,department_id
FROM employees
WHERE department_id MOD 20 = 0;#选择工资不在5000到12000的员工的姓名和工资
SELECT last_name,salary
FROM employees
#where salary < 5000 or salary > 12000;
WHERE salary NOT BETWEEN 5000 AND 12000;
#where !(salary >= 5000 and salary <= 12000);#选择公司中没有管理者的员工姓名及job_id
SELECT last_name,job_id
FROM employees
WHERE manager_id IS NULL;

#04-排序和分页#1. 排序
# 1.1
#按照员工的工资从高到低排序
# desc: descend,降序
# asc : ascend,升序
SELECT employee_id,salary
FROM employees
#order by salary desc; #降序
#order by salary asc; #升序
ORDER BY salary ;#如果没有显式指名asc或desc，则默认升序排列# 1.2 使用列的别名进行排序
#别名可以在order by中使用
SELECT employee_id,last_name,salary sal
FROM employees
ORDER BY sal;#不能在过滤条件中使用列的别名。
#如下操作报错：
SELECT employee_id,last_name,salary sal
FROM employees
WHERE sal >= 6000;#order by要声明在where的后面
SELECT employee_id,last_name,salary sal
FROM employees
WHERE salary > 5000
ORDER BY last_name ASC; #1.3 二级排序
SELECT employee_id,last_name,department_id,salary
FROM employees
ORDER BY department_id,salary DESC;#2. 分页
#每页显示20条记录，显示第1页数据：
SELECT employee_id,last_name,salary
FROM employees
LIMIT 0,20;#每页显示20条记录，显示第2页数据：
SELECT employee_id,last_name,salary
FROM employees
LIMIT 20,20;#每页显示20条记录，显示第3页数据：
SELECT employee_id,last_name,salary
FROM employees
LIMIT 40,20;#每页显示pageSize条记录，显示第pageNo页数据：
#limit (pageNo - 1) * pageSize,pageSize#查询工资最高的20个员工信息:top-N
SELECT employee_id,last_name,salary
FROM employees
ORDER BY salary DESC
LIMIT 0,20; #limit 要声明在order by的后面

#05-多表查询#出现了笛卡尔积的错误
SELECT employee_id,last_name,department_name
FROM employees,departments; #查询出2889行数据SELECT 2889/107
FROM DUAL;SELECT *
FROM departments;#正确的写法:多表的查询，一定要有连接条件
SELECT employee_id,last_name,department_name
FROM employees,departments
#多表的连接条件
WHERE employees.`department_id` = departments.`department_id`#进一步：如果查询的字段在多个表中都出现，则一定需要指明来自于哪个表。比如：department_id
SELECT employee_id,last_name,department_name,departments.department_id
FROM employees,departments
WHERE employees.`department_id` = departments.`department_id`#建议开发中多表查询中，查询的字段都指明来自于哪个表。--->sql优化
SELECT employees.employee_id,employees.last_name,departments.department_name,departments.department_id
FROM employees,departments
WHERE employees.`department_id` = departments.`department_id`#表的别名的使用。一旦给表起了别名，就可以在select中或where中使用。
SELECT e.employee_id,e.last_name,d.department_name,d.department_id
FROM employees e,departments d
WHERE e.`department_id` = d.`department_id`#需求：
SELECT e.employee_id,e.last_name,d.department_name,l.city
FROM employees e,departments d,locations l
#连接条件
WHERE e.department_id = d.department_id
AND d.location_id = l.location_id;#结论：如果实现n个表的多表查询，则至少需要n - 1个连接条件。 ##############################################
/*
多表查询的分类：
1. 等值连接  vs  不等值连接
2. 自连接    vs  非自连接
3. 内连接    vs  外连接*/#不等值连接
SELECT employee_id,salary,grade_level
FROM employees e,job_grades j
#where e.`salary` >= j.`lowest_sal` and e.`salary` <= j.`highest_sal`;
WHERE e.`salary` BETWEEN j.`lowest_sal` AND j.`highest_sal`;#自连接
#查询员工的employee_id,last_name及其管理者的employee_id,last_name
SELECT emp.employee_id,emp.last_name,mgr.employee_id,mgr.last_name
FROM employees emp,employees mgr
WHERE emp.`manager_id` = mgr.`employee_id`;########################################################
/*
内连接：合并具有同一列的两个以上的表的行, 结果集中不包含一个表与另一个表不匹配的行
外连接：分为左外连接 和 右外连接左外连接：两个表在连接过程中除了返回满足连接条件的行以外，还返回左表中不满足条件的行这种连接称为左外连接。右外连接：两个表在连接过程中除了返回满足连接条件的行以外，还返回右表中不满足条件的行这种连接称为右外连接。
*/
#内连接的例子：
SELECT employee_id,last_name,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.`department_id`# 要想实现外连接，需要使用sql-99语法中的相关结构。#####################################################sql-99语法实现内连接：举例1
SELECT employee_id,last_name,department_name
FROM employees e INNER JOIN departments d
ON e.`department_id` = d.`department_id`;
#举例2：
SELECT employee_id,last_name,department_name,city
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`
JOIN locations l
ON d.`location_id` = l.`location_id`;#左外连接：
#需求：查询所有员工的employee_id,last_name,department_name
SELECT employee_id,last_name,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`;#右外连接：
SELECT employee_id,last_name,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`;#与上一个select是相同的需求。
SELECT employee_id,last_name,department_name
FROM departments d LEFT JOIN employees e
ON e.`department_id` = d.`department_id`;# 满外连接：两个表在连接过程中除了返回满足连接条件的行以外，还返回左表和右表中不满足条件的行#这种连接称为满外连接。
# mysql 不支持full join
SELECT employee_id,last_name,department_name
FROM employees e FULL JOIN departments d
ON e.`department_id` = d.`department_id`;#课后练习# 2.查询90号部门员工的job_id和90号部门的location_id
SELECT e.job_id,d.location_id
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.department_id = 90;# 3.选择所有有奖金的员工的 last_name , department_name , location_id , city
SELECT e.last_name , d.department_name , d.location_id , l.city
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
LEFT JOIN locations l
ON d.`location_id` = l.`location_id`
WHERE e.`commission_pct` IS NOT NULL;# 4.选择city在Toronto工作的员工的 last_name , job_id , department_id , department_name
SELECT last_name , job_id , e.department_id , department_name
FROM employees e
JOIN departments d
ON e.`department_id` = d.`department_id`
JOIN locations l
ON l.`location_id` = d.`location_id`
WHERE l.`city` = 'Toronto';# 5.选择所有员工的姓名，员工号，以及他的管理者的姓名和员工号，结果类似于下面的格式
employees   Emp#    manager Mgr#
kochhar     101 king    100SELECT emp.last_name employees, emp.employee_id "Emp#", mgr.last_name manager, mgr.employee_id "Mgr#"
FROM employees emp
LEFT JOIN employees mgr
ON emp.manager_id = mgr.employee_id;

#06-7种JOIN
#中图：内连接   106条
SELECT employee_id,last_name,department_name
FROM employees e INNER JOIN departments d
ON e.`department_id` = d.`department_id`;#左上图：左外连接  107条
SELECT employee_id,last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;#右上图：右外连接  122条
SELECT employee_id,last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;#左中图： 1条
SELECT employee_id,last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL;#右中图：
SELECT employee_id,last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;#左下图：满外连接  123条
#方式一：左中图 + 右上图
SELECT employee_id,last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL
UNION ALL
SELECT employee_id,last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`#方式二：左上图 + 右中图 :略#右下图：左中图 + 右中图： 17条记录SELECT employee_id,last_name,department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL
UNION ALL
SELECT employee_id,last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;#结论：能使用union all的场景，就不推荐使用union。因为避免去重，效率低。

#07-单行函数
# 1. 字符串类型
SELECT CONCAT('hello','world','hello','beijing') "details"
FROM DUAL;#需求： xxx worked for yyySELECT CONCAT(emp.last_name,' worked for ', mgr.last_name) "details"
FROM employees emp JOIN employees mgr
ON emp.`manager_id` = mgr.`employee_id`;SELECT CONCAT_WS('-','hello','world','beijing')
FROM DUAL;SELECT CHAR_LENGTH('hello'),LENGTH('hello'),CHAR_LENGTH('中国'),LENGTH('中国')
FROM DUAL;
#sql中索引从1开始！
SELECT INSERT('helloworld',2,3,'aaaaa')
FROM DUAL;#下面的查询中，salary会自动的转换为字符串类型，此自动转换的行为称为：隐式转换
SELECT employee_id,LPAD(salary,10,' ') "details", RPAD(last_name,10,' ')
FROM employees;DESC employees;# 针对于数值类型、字符串类型、日期类型存在隐式转换。
SELECT 1 + '1'
FROM DUAL;SELECT TRIM('aa' FROM 'aaaahelaaloa')
FROM DUAL;SELECT REPEAT('hello',5)
FROM DUAL;SELECT STRCMP('abc','abe')
FROM DUAL;
#索引从1 开始
SELECT SUBSTRING('hello',2,2)
FROM DUAL;# 2. 数值类型
SELECT CEIL(123.342),FLOOR(23.999),
MOD(12,5),MOD(12,-5),MOD(-12,5),MOD(-12,-5),RAND() * 100
FROM DUAL;SELECT ROUND(123.567),ROUND(123.567,0),ROUND(123.567,1),ROUND(123.567,-2)
FROM DUAL;SELECT TRUNCATE(123.967,0),TRUNCATE(123.567,1),TRUNCATE(123.567,-1)
FROM DUAL;# 3. 日期类型
SELECT employee_id,hire_date
FROM employees
WHERE hire_date = '1993-01-13';#如何获取当前的年月日、时分秒、年月日时分秒
SELECT CURDATE(),CURRENT_DATE(),CURTIME(),CURRENT_TIME(),NOW(),SYSDATE()
FROM DUAL;/*
insert into employees(employee_id,last_name,hire_date)
values(304,'Tom',curdate())
*/SELECT YEAR(CURDATE()),MONTH(CURDATE()),DAY(CURDATE()),
HOUR(CURTIME()),MINUTE(CURTIME()),SECOND(CURTIME())
FROM DUAL;SELECT DAYOFWEEK(CURDATE()),WEEKDAY(NOW()),DAYNAME(CURDATE())
FROM DUAL;SELECT DATE_ADD(NOW(), INTERVAL 1 YEAR)
FROM DUAL;SELECT DATE_ADD(NOW(), INTERVAL -1 YEAR);   #可以是负数SELECT DATE_ADD(NOW(), INTERVAL '1_1' YEAR_MONTH);   #需要单引号#显式操作
#格式化：日期 ---> 字符串
#DATE_FORMAT(datetime ,fmt)
SELECT DATE_FORMAT(NOW(),'%Y/%M/%d %h:%i:%s')
FROM DUAL;#解析： 字符串 ---> 日期
#STR_TO_DATE(str, fmt)
SELECT STR_TO_DATE('2021/May/26 09:54:17','%Y/%M/%d %h:%i:%s')
FROM DUAL;#隐式操作：日期类型、字符串类型、数值类型之间存在隐式的转换。# 4. 流程控制
#结构1 ： if
SELECT employee_id,last_name,salary,IF(salary > 10000,'高工资','低工资') "details",
IF(commission_pct IS NOT NULL,commission_pct,0) "details1"
FROM employees;#结构2： ifnull
SELECT employee_id,last_name,salary,
IF(commission_pct IS NOT NULL,commission_pct,0) "details",
IFNULL(commission_pct,0) "details1",
salary * (1 + commission_pct),salary * (1 + IFNULL(commission_pct,0))
FROM employees; #结构3：case when ... then ... when ... then ... else ... end 类似于if-else if - .. -else 的多选一SELECT employee_id,last_name,salary,CASE WHEN salary > 15000 THEN '高富帅'WHEN salary > 10000 THEN '潜力股'WHEN salary > 5000 THEN '打工人'ELSE '小屌丝' END "details"
FROM employees; SELECT employee_id,last_name,department_id,CASE WHEN department_id = 10 THEN '10号部门'WHEN department_id = 20 THEN '20号部门'WHEN department_id = 30 THEN '30号部门'END "details"
FROM employees;#结构4：case ... when ... then ... when ... then ... else ... end 类似于switch-case
SELECT employee_id,last_name,department_id,CASE department_id WHEN 10 THEN '10号部门'WHEN 20 THEN '20号部门'WHEN 30 THEN '30号部门'ELSE '其他部门' END "details"
FROM employees;/*
**练习：查询部门号为 10,20, 30 的员工信息,
若部门号为 10, 则打印其工资的 1.1 倍,
20 号部门, 则打印其工资的 1.2 倍,
30 号部门打印其工资的 1.3 倍数。
*/SELECT employee_id,last_name,salary,department_id,CASE department_id WHEN 10 THEN salary * 1.1WHEN 20 THEN salary * 1.2WHEN 30 THEN salary * 1.3END "details"
FROM employees
WHERE department_id IN (10,20,30);# 5. 其它函数
SELECT DATABASE(),VERSION(),USER()
FROM DUAL;SELECT PASSWORD('abcd'),MD5(MD5('abcd'))
FROM DUAL;

#08-分组函数#上篇：常见的5个组函数：avg() / sum() / min() / max() / count()#1. avg() / sum(): 只适用于数值类型的字段
SELECT AVG(salary),SUM(salary)#,avg(last_name),sum(last_name),sum(hire_date)
FROM employees;#2. min()/ max():适用于数值类型、字符串类型、日期类型的字段
SELECT MIN(salary),MAX(salary),MIN(employee_id),MIN(last_name),MIN(hire_date),MAX(hire_date)
FROM employees;#3.count(): 适用于数值类型、字符串类型、日期类型的字段
# 在计算个数时，null值不考虑在内。
SELECT COUNT(salary),COUNT(last_name),COUNT(hire_date),COUNT(commission_pct)
FROM employees;#需求：查询员工表中员工的个数
SELECT COUNT(employee_id),COUNT(last_name),COUNT(1),COUNT(2),COUNT(*)
FROM employees;DESC employees;#count 与 avg 、 sum 三者的关系: avg = sum / count
SELECT AVG(salary),SUM(salary)/COUNT(salary),
AVG(commission_pct),SUM(commission_pct)/COUNT(commission_pct),
SUM(commission_pct)/107
FROM employees;#需求：查询员工的平均奖金级别？
SELECT SUM(commission_pct)/COUNT(IFNULL(commission_pct,10))
FROM employees;#下篇：group by / having #查询公司的平均工资
SELECT AVG(salary)
FROM employees;#查询各个部门的平均工资
SELECT department_id,AVG(salary)
FROM employees
GROUP BY department_idSELECT job_id,AVG(salary)
FROM employees
GROUP BY job_id;SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id,job_id;#与上一个需求执行的结果是相同的
SELECT job_id,department_id,AVG(salary)
FROM employees
GROUP BY job_id,department_id;####
#如下的语句是正确的：
SELECT AVG(salary)
FROM employees
GROUP BY department_id#错误的：
SELECT department_id,AVG(salary)
FROM employees;#结论：select中出现了组函数和非组函数的字段，那么非组函数的字段一定要声明在group by中。
#反之，声明在group by中的字段，可以不声明在select中。SELECT department_id,AVG(salary)
FROM employees
GROUP BY department_id,job_id;#需求：查询部门最高工资比10000高的部门及其最高工资
#错误的
SELECT department_id,MAX(salary)
FROM employees
WHERE MAX(salary) > 10000
GROUP BY department_id #结论：如果过滤条件中出现了组函数，则需要使用having替换where实现过滤
SELECT department_id,MAX(salary)
FROM employees
GROUP BY department_id
HAVING MAX(salary) > 10000#需求：查询10,20,30,40号部门中，部门最高工资比10000高的部门及其最高工资
#写法一：推荐！优于写法二，因为效率高
#结论：如果过滤条件1中出现了组函数，那么将过滤条件1声明在having中
#      如果过滤条件2中没有出现组函数，推荐将过滤条件2声明在where中。
SELECT department_id,MAX(salary)
FROM employees
WHERE department_id IN (10,20,30,40)
GROUP BY department_id
HAVING MAX(salary) > 10000#写法二：
SELECT department_id,MAX(salary)
FROM employees
GROUP BY department_id
HAVING MAX(salary) > 10000
AND department_id IN (10,20,30,40);/*
小结：
#查询语句的结构1：SELECT ...,....,...,....
FROM ....,...,....
WHERE 多表的连接条件 AND 非组函数的过滤条件
GROUP BY ...,....
HAVING 组函数的过滤条件
ORDER BY ... ASC/DESC,... ASC/DESC
LIMIT ...,....#查询语句的结构2：
SELECT ...,...,...
FROM ... (LEFT OUTER / RIGHT OUTER )JOIN ... ON ...
(LEFT OUTER / RIGHT OUTER )JOIN ... ON ....
WHERE 非组函数的过滤条件
GROUP BY ...,....
HAVING 组函数的过滤条件
ORDER BY ... ASC/DESC,... ASC/DESC
LIMIT ...,....*/

#09-子查询
#谁的工资比Abel的高？#方式1：
SELECT salary
FROM employees
WHERE last_name = 'Abel';SELECT last_name,salary
FROM employees
WHERE salary > 11000;#方式2：自连接
SELECT e1.last_name,e1.salary
FROM employees e1 JOIN employees e2
ON e1.`salary` > e2.`salary`
WHERE e2.`last_name` = 'Abel';#方式3：子查询
SELECT last_name,salary
FROM employees
WHERE salary > (SELECT salaryFROM employeesWHERE last_name = 'Abel')# 子查询的概念： 外层：外查询、主查询 ; 内层：内查询、子查询
# 子查询的分类： 单行子查询、 多行子查询#1. 单行子查询
# 可以使用的比较运算符有： =   >   >=  <  <=  <>  !=#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 last_name,job_id,salary
FROM employees
WHERE job_id = (SELECT job_idFROM employeesWHERE employee_id = 141)
AND salary > (SELECT salaryFROM employeesWHERE employee_id = 143);#题目：返回公司工资最少的员工的last_name,job_id和salarySELECT last_name,job_id,salary
FROM employees
WHERE salary = (SELECT MIN(salary)FROM employees);#技巧：书写子查询的方式：①从外往里写 ②从里往外写#题目：查询最低工资大于60号部门最低工资的部门id和其最低工资SELECT department_id,MIN(salary)
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary ) > (SELECT MIN(salary)FROM employeesWHERE department_id = 60)#空值的情况：
SELECT last_name, job_id
FROM   employees
WHERE  job_id =(SELECT job_idFROM   employeesWHERE  last_name = 'Haas');#非法使用子查询
SELECT employee_id, last_name
FROM   employees
WHERE  salary =(SELECT   MIN(salary)FROM     employeesGROUP BY department_id);#2. 多行子查询#多行子查询可以使用的比较操作符有：in  all any
#in:
SELECT employee_id, last_name
FROM   employees
WHERE  salary IN(SELECT   MIN(salary)FROM     employeesGROUP BY department_id);#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,salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ANY(SELECT salaryFROM employeesWHERE 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,salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ALL(SELECT salaryFROM employeesWHERE job_id = 'IT_PROG')      ###################################################
#题目：查询员工中工资大于公司平均工资的员工的last_name,salary和其department_id
SELECT last_name,salary,department_id
FROM employees
WHERE salary > (SELECT AVG(salary)FROM employees);#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 last_name,salary,department_id
FROM employees e1
WHERE salary > (#本部门的平均工资SELECT AVG(salary)FROM employees e2WHERE e2.department_id = e1.`department_id`);#方式二：除了在group by 和limit 之外的位置都可以编写子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(SELECT department_id,AVG(salary) avg_salFROM employeesGROUP BY department_id) dept_avg_sal
WHERE e.department_id = dept_avg_sal.department_id
AND e.`salary` > dept_avg_sal.avg_sal;#另例：查询员工的employee_id,last_name,要求按照department_name从小到大排序SELECT employee_id,last_name
FROM employees e
ORDER BY (SELECT department_nameFROM departments dWHERE e.`department_id` = d.`department_id`) DESC;#EXISTS的使用
#题目：查询公司管理者的employee_id,last_name,job_id,department_id信息
#方式一：
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN (SELECT DISTINCT manager_idFROM employees);#方式二：
SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (SELECT 'x'FROM employees e2WHERE e1.`employee_id` = e2.manager_id);

#10-子查询课后练习#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,last_name
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
#自连接
SELECT emp.last_name,emp.salary
FROM employees emp JOIN employees mgr
ON emp.`manager_id` = mgr.`employee_id`
WHERE mgr.last_name = 'King';#子查询
SELECT last_name,salary
FROM employees
WHERE manager_id IN (SELECT employee_idFROM employeesWHERE last_name = 'King');#8.查询平均工资最低的部门信息
#方式一：
SELECT *
FROM departments
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) = (SELECT MIN(avg_sal)FROM (SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id) dept_avg_sal))          #方式二：
SELECT *
FROM departments
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) <= ALL(SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id) )#方式三：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          ))#方式四：
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（难）
#方式一：
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;#方式二：
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.department_id) "avg_sal"
FROM departments d
WHERE department_id = (SELECT department_idFROM employeesGROUP BY department_idHAVING AVG(salary) <= ALL(SELECT AVG(salary) avg_salFROM employeesGROUP BY department_id) )#10.查询平均工资最高的 job 信息#11.查询平均工资高于公司平均工资的部门有哪些?#12.查询出公司中所有 manager 的详细信息.#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?#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

#11-创建和管理表 --DDL#数据库对象：表、视图、存储过程、函数、触发器、同义词、索引等# 1.创建库
CREATE DATABASE database0419;SHOW DATABASES; #查看所有的数据库# 2. 使用指定的数据库
USE database0419;SHOW TABLES; #查看指定的库下包含哪些表USE temp;SHOW TABLES; SELECT *
FROM employees;# 3. 删除库
DROP DATABASE database0419;# 4. 创建表
# 方式一："白手起家"的方式
CREATE TABLE emp1(
id INT,
last_name VARCHAR(15),
email VARCHAR(25),
salary DOUBLE(10,2),
hire_date DATE
);DESC emp1;SELECT *
FROM emp1;#方式二：基于现有的表，创建新的表
CREATE TABLE emp2
AS
SELECT employee_id,last_name,salary
FROM employees ;DESC emp2;
DESC employees;
#说明：使用此种方式创建表时，还可以将原有表中的数据复制到新表中。
SELECT *
FROM emp2;
#如下结构中select中列的别名，就作为新创建的表的字段名
CREATE TABLE emp3
AS
SELECT employee_id emp_id,last_name,salary AS sal
FROM employees ;#查询失败
SELECT employee_id
FROM emp3;#练习1：复制employees表，包含所有数据
CREATE TABLE employees_copy
AS
SELECT *
FROM employees;SELECT *
FROM employees_copy;SELECT *
FROM employees;#练习2: 复制employees表，不包含任何数据
CREATE TABLE employees_copy_blank
AS
SELECT *
FROM employees
WHERE 1 = 2;SELECT *
FROM employees_copy_blank;# 5. 修改表
DESC emp3;# 5.1 增加一个列
ALTER TABLE emp3
ADD email VARCHAR(25);SELECT *
FROM emp3;# 5.2 删除一个列
ALTER TABLE emp3
DROP email;ALTER TABLE emp3
DROP COLUMN sal;# 5.3 修改字段(类型、储值范围)
DESC emp3;ALTER TABLE emp3
MODIFY last_name VARCHAR(30);#通常不会修改字段的类型！！
#报错
ALTER TABLE emp3
MODIFY last_name INT;# 5.4 重命名字段
DESC emp3;ALTER TABLE emp3
CHANGE last_name lname VARCHAR(21);# 6. 重命名表
RENAME TABLE emp3 TO myemp3;SELECT *
FROM myemp3;# 7. 删除表
DROP TABLE employees_copy;# 8. 清空表: 清空表中的数据，但是表结构保留SELECT *
FROM myemp3;TRUNCATE TABLE myemp3;/*
对比TRUNCATE TABLE 和 DELETE FROM ...
结论：TRUNCATE TABLE 一旦操作，就不可以回滚数据DETELE FROM  支持删除表中的所有数据，可以回滚数据测试：COMMIT 与 ROLLBACK的使用
1. COMMIT:表示提交数据。数据一旦被提交，就不可回滚。ROLLBACK:表示回滚数据。 回滚操作，也只能回滚到最近的一次COMMIT之后。2. 默认情况下，对数据表的操作(DDL、DML),都是在执行之后，默认提交数据的。要想测试TRUNCATE TABLE 和 DETELE FROM的区别，需要关闭默认提交的行为：SET autocommit = FALSE; */CREATE TABLE myemp
AS
SELECT *
FROM employees;SELECT *
FROM myemp;COMMIT;#首先测试DELETE FROM
SET autocommit = FALSE;DELETE FROM myemp; #删除数据SELECT *
FROM myemp;ROLLBACK; #回滚数据#接着测试TRUNCATE TABLE
COMMIT;SET autocommit = FALSE;TRUNCATE TABLE myemp; #清空表SELECT *
FROM myemp;#并没有将数据回滚成功
ROLLBACK; #回滚数据#结论：以TRUNCATE TABLE为代表的DDL操作，都会在执行完以后，自动的COMMIT提交数据。
#   而且此提交行为不受SET autocommit = FALSE;的影响。所以，ROLLBACK行为对DDL操作都失效。# 关于COMMIT、ROLLBACK涉及到数据库事务的操作

#12-数据处理之增、删、改# 1. 增/添加:INSERT#方式1：一条一条的添加
DESC emp1;SELECT *
FROM emp1;INSERT INTO emp1
VALUES(1,'Tom','tom@126.com',3000,CURDATE());INSERT INTO emp1
VALUES(2,'Tom1','tom@126.com',NULL,CURDATE());#进化一步：
INSERT INTO emp1(id,last_name,salary,hire_date,email)
VALUES(3,'Tom2',5000,'2000-10-12','Tom2@126.com');#没有声明的字段，在添加操作执行完后，值为null
INSERT INTO emp1(id,last_name,email)
VALUES(4,'Tom3','Tom3@126.com');#方式2：基于现有的表
INSERT INTO emp1(id,last_name)
SELECT employee_id,last_name
FROM employees
WHERE department_id IN (10,20,30)SELECT *
FROM emp1;DESC emp1;
DESC employees;#2. 删除数据:delete from .... where ...
DELETE FROM emp1
WHERE id = 1;#3. 修改数据: update ... set ..,.. where ...UPDATE emp1
SET salary = 7000
WHERE id = 4;UPDATE emp1
SET salary = 8000,hire_date = CURDATE()
WHERE id = 4;

#13-约束（constraint)DESC employees;
/*
1. 约束，针对表中的数据，在添加、删除、修改的过程中，进行的限制。2. 约束的分类:
角度一（从声明的位置上）：列级约束  vs 表级约束
角度二（从作用的列的数量上）：单列约束  vs 多列约束
角度三（从功能上区分）：not null :非空约束unique: 唯一性约束primary key:主键约束foreign key:外键约束check : 检查约束default : 默认值约束3. 设置约束的时机：情况1：在CREATE TABLE 的同时，给表的字段添加上约束。情况2：通过 ALTER TABLE 的方式添加、删除约束
*/# 1. not null :非空约束
# 创建表的同时，添加约束
CREATE TABLE  emp3(
id INT NOT NULL,
last_name VARCHAR(15) NOT NULL,
email VARCHAR(25),
hire_date DATE
);DESC emp3;
#添加成功
INSERT INTO emp3(id,last_name,email,hire_date)
VALUES(1,'Tom','tom@126.com',CURDATE());SELECT * FROM emp3;#添加失败
INSERT INTO emp3(last_name,email,hire_date)
VALUES('Tom','tom@126.com',CURDATE());#添加失败
INSERT INTO emp3(id,last_name,email,hire_date)
VALUES(2,NULL,'tom@126.com',CURDATE());#在ALTER TABLE时，删除非空约束
ALTER TABLE emp3
MODIFY last_name VARCHAR(15) NULL;DESC emp3;#添加成功
INSERT INTO emp3(id,last_name,email,hire_date)
VALUES(2,NULL,'tom@126.com',CURDATE());SELECT *
FROM emp3;#在ALTER TABLE时，添加非空约束
ALTER TABLE emp3
MODIFY hire_date DATE NOT NULL;#2. unique: 唯一性约束
# 创建表的同时，添加约束
CREATE TABLE  emp4(
id INT UNIQUE, #列级约束
last_name VARCHAR(15),
email VARCHAR(25),
hire_date DATE,
#表级约束
CONSTRAINT emp4_email_uk UNIQUE(email)
);DESC emp4;
#添加成功
INSERT INTO emp4(id,last_name,email,hire_date)
VALUES(1,'Tom','tom@126.com',CURDATE());SELECT * FROM emp4;#添加失败
INSERT INTO emp4(id,last_name,email,hire_date)
VALUES(1,'Tom','tom@126.com',CURDATE());#添加失败
INSERT INTO emp4(id,last_name,email,hire_date)
VALUES(2,'Tom','tom@126.com',CURDATE());#添加成功
INSERT INTO emp4(id,last_name,email,hire_date)
VALUES(2,'Tom1',NULL,CURDATE());#添加成功
INSERT INTO emp4(id,last_name,email,hire_date)
VALUES(3,'Tom1',NULL,CURDATE());
#结论：声明为unique约束的字段，在添加或修改数据时，允许多次设置为null./*
在修改表的时候，如何删除约束？1. 在创建唯一约束的时候，如果不给唯一约束名称，就默认和列名相同
2. MySQL会给唯一约束的列上默认创建一个唯一索引
3. 删除唯一约束只能通过删除唯一索引的方式删除
4. 删除时需要指定唯一索引名，唯一索引名就和唯一约束名一样。
5. 如果创建唯一约束时未指定名称，如果是单列，就默认和列名相同，如果是组合列，那么默认和()中排在第一个的列名相同。也可以自定义唯一性约束名。
*/DESC emp4;
#删除索引，进而删除了唯一性约束
ALTER TABLE emp4
DROP INDEX emp4_email_uk;ALTER TABLE emp4
DROP INDEX id;#添加唯一性约束
ALTER TABLE emp4
ADD CONSTRAINT emp4_id_uk UNIQUE(id);#3. primary key:主键约束
# 一个表中只能声明一个主键约束
# 主键约束，既满足唯一性，也满足非空性。
# 通过声明有主键约束的字段，可以确定表中的唯一的一条记录。
# 通常，在创建表的同时，都需要指名一个主键约束。CREATE TABLE emp5(
id INT PRIMARY KEY, #列级约束
last_name VARCHAR(15),
email VARCHAR(25),
hire_date DATE,
salary DOUBLE(10,2)
);DESC emp5;
#添加成功
INSERT INTO emp5(id,last_name,email,salary)
VALUES(1,'Tom','tom@126.com',2000);SELECT *
FROM emp5;#添加失败
INSERT INTO emp5(id,last_name,email,salary)
VALUES(1,'Tom','tom@126.com',2000);#添加失败
INSERT INTO emp5(id,last_name,email,salary)
VALUES(NULL,'Tom','tom@126.com',2000);#开发中常见的声明方式：
CREATE TABLE emp6(
id INT AUTO_INCREMENT,
last_name VARCHAR(15),
email VARCHAR(25),
hire_date DATE,
salary DOUBLE(10,2),
#表级约束
CONSTRAINT emp6_id_pk PRIMARY KEY(id)
);INSERT INTO emp6(last_name,email,salary)
VALUES('Tom','tom@126.com',2000);SELECT *
FROM emp6;#如何删除主键
ALTER TABLE emp5
DROP PRIMARY KEY;DESC emp5;#添加主键约束
ALTER TABLE emp5
ADD CONSTRAINT emp5_id_pk PRIMARY KEY(id);#4. foreign key:外键约束
#作用：在表A的字段a上声明有一个外键约束，与表B中的字段b相关联。则字段a在insert等操作时，
#     其赋的值一定是字段b中出现过的数据。#要求：要想能关联成功，必须要求字段b声音有主键约束或唯一性约束CREATE TABLE dept7(
dept_id INT,
dept_name VARCHAR(10)
);#添加外键约束失败
CREATE TABLE emp7(
id INT,
last_name VARCHAR(15),
dept_id INT,#声明外键:表级约束
CONSTRAINT emp7_dept_id_fk FOREIGN KEY(dept_id) REFERENCES dept7(dept_id)
);#补救措施：
ALTER TABLE dept7
ADD CONSTRAINT dept7_dept_id_pk PRIMARY KEY(dept_id);DESC dept7;#添加外键约束成功
CREATE TABLE emp7(
id INT,
last_name VARCHAR(15),
dept_id INT,#声明外键:表级约束
CONSTRAINT emp7_dept_id_fk FOREIGN KEY(dept_id) REFERENCES dept7(dept_id)
);
#添加失败
INSERT INTO emp7(id,last_name,dept_id)
VALUES(1,'Tom',10);#
INSERT INTO dept7(dept_id,dept_name)
VALUES(10,'IT');#添加成功
INSERT INTO emp7(id,last_name,dept_id)
VALUES(1,'Tom',10);SELECT *
FROM emp7;#结论：在实际开发中，不建议在创建表时使用外键约束。#5. 检查约束(check)
# 对mysql失效CREATE TABLE emp8(
id INT,
last_name VARCHAR(15),
salary DOUBLE(10,2) CHECK(salary > 3000));DESC emp8;INSERT INTO emp8
VALUES(1,'Tom',4000);
#添加成功
INSERT INTO emp8
VALUES(2,'Tom1',2000);SELECT *
FROM emp8;#6.默认值约束（default）CREATE TABLE emp9(
id INT,
last_name VARCHAR(15),
salary DOUBLE(10,2) DEFAULT 2500);DESC emp9;INSERT INTO emp9
VALUES(1,'Tom',5000);INSERT INTO emp9(id,last_name)
VALUES(1,'Tom');SELECT *
FROM emp9;

三天学会MySQL -宋红康相关推荐

1. MySQL数据库教程天花板，mysql安装到mysql高级，强|硬 宋红康版

   MySQL数据库教程天花板,mysql安装到mysql高级,强|硬 宋红康版(自用不可外传) 文章目录 MySQL数据库笔记 第一部分 MySQL基础篇 第01章 数据库概述 1. 为什么要使用数据库 ...

2. 【宋红康 MySQL数据库 】【高级篇】【12】性能分析工具的使用

   持续学习&持续更新中- 学习态度:守破离 [宋红康 MySQL数据库 ][高级篇][12]性能分析工具的使用 数据库服务器的优化步骤 查看系统性能参数 统计SQL的查询成本:last_quer ...

3. 【宋红康 MySQL数据库】【00】课程大纲

   持续学习&持续更新中- 学习态度:守破离 [宋红康 MySQL数据库][00]课程大纲 MySQL数据库基础篇大纲 1. 数据库概述与MySQL安装篇 2. SQL之SELECT使用篇 3. ...

4. 【MySQL入门到高级之基础篇(参考尚硅谷宋红康老师2022版)】

   文章目录 第一章数据库概述 为什么要使用数据库 数据库与数据库管理系统 数据库的相关概念 数据库与数据库管理系统的关系 常见的数据库管理系统排名(DBMS) 常见的数据库介绍 MySQL介绍 概述 M ...

5. 【宋红康 MySQL数据库】【01】数据库概述

   持续学习&持续更新中- 学习态度:守破离 [宋红康 MySQL数据库][01]数据库概述 1. 为什么要使用数据库 2. 数据库与数据库管理系统 2.1 数据库的相关概念 2.2 数据库与数据 ...

6. 【宋红康 MySQL数据库 】【高级篇】【17】MySQL事务日志：redo、undo

   持续学习&持续更新中- 学习态度:守破离 [宋红康 MySQL数据库 ][高级篇][17]MySQL事务日志 概述 redo日志 为什么需要redo日志 redo日志的好处.特点 redo的组 ...

7. 【宋红康 MySQL数据库 】【高级篇】【07】MySQL的存储引擎

   持续学习&持续更新中- 学习态度:守破离 [宋红康 MySQL数据库 ][高级篇][07]MySQL的存储引擎 存储引擎 查看存储引擎 设置系统默认的存储引擎 设置表的存储引擎 引擎介绍 In ...

8. 尚硅谷-宋红康-MySQL高级性能篇

   尚硅谷-宋红康-MySQL高级性能篇 第1章 Linux下MySQL的安装与使用 1. 安装前说明 1.1 Linux系统及工具准备 二级目录 三级目录 第1章 Linux下MySQL的安装与使用 1 ...

9. 【宋红康 MySQL数据库】【03】SQL概述_常见的数据库对象

   持续学习&持续更新中- 学习态度:守破离 [宋红康 MySQL数据库][03]SQL概述_常见的数据库对象 SQL概述 什么是SQL SQL背景知识 SQL分类 DDL(Data Defini ...

最新文章

1. BCH(比特币现金)的货币流通速度是BTC的6倍
2. 5 步助你成为一名优秀的 Docker 代码贡献者
3. MySQL复习资料(二)——MySQL-DDL语句
4. 从实例到数理来解析感知机学习算法(PLA)
5. linux 两个驱动 竞态,第7章　Linux设备驱动中的并发控制之一（并发与竞态）
6. postgresql 数据库
7. TransactionScope oracle不能用的问题（转载）
8. 中教云教师备课云平台获北京市新技术新产品（服务）认定
9. oracle exp执行失败,EXP-00056: 遇到 ORACLE 错误 25153
10. linux下串口工具minicom
11. PMP报考 你成功了吗？
12. vue 使用flowplayer_Flowplayer视频播放插件
13. 水利系统防雷，如何做好洪水预报警报系统的雷电防护
14. 计算机读不了硬盘分区,修复移动硬盘分区故障和无法识别计算机
15. 2020-12-28 微信支付二面
16. linux下线程池实现
17. 20条不该打破的设计规则
18. 单链表建立的两种方法 头插法和尾插法
19. Python基于Oxford-IIIT Pet Dataset实现宠物识别系统
20. 怎么做好网络营销推广引流客户？

热门文章

1. 目标检测 R-CNN讲解ppt
2. CImageList图标大小
3. Java基础语法变量的使用
4. 按钮，干簧管，霍尔，声音，震动，倾斜，红外开关小谈
5. python爬取b站数据
6. Redhat7.3安装步骤
7. PCB设计铝电解电容中必须要点
8. 改进的 A*算法的路径规划（路径规划+代码+毕业设计）
9. HBASE分布式安装
10. RGB颜色转换十六进制颜色