【MySQL】基础篇-SELECT语言、运算符、排序与分页、多表查询-摩杜云开发者社区

0、基础篇

SHOW DATABASES;

SHOW TABLES;

use db_learn;

CREATE TABLE user (id int, name VARCHAR(10));
INSERT into user VALUES(100, "s1");
INSERT into user VALUES(101, "沈1");
SELECT * FROM user;

SHOW CREATE TABLE user;
-- 结果
CREATE TABLE `user` (
  `id` int DEFAULT NULL,
  `name` varchar(10) COLLATE utf8mb4_general_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci

SHOW VARIABLES LIKE "character_%";
-- 结果 
character_set_client	utf8mb4
character_set_connection	utf8mb4
character_set_database	utf8mb4
character_set_filesystem	binary
character_set_results	utf8mb4
character_set_server	utf8mb4
character_set_system	utf8mb3
character_sets_dir	/usr/local/mysql-8.1.0-macos13-arm64/share/charsets/

SHOW VARIABLES LIKE "collation_%";
-- 结果
collation_connection	utf8mb4_0900_ai_ci
collation_database	utf8mb4_general_ci
collation_server	utf8mb4_0900_ai_ci

1、基本的SELECT语句

1. SQL的分类

DDL:数据定义语言。CREATE \ ALTER \ DROP \ RENAME \ TRUNCATE
-.DML:数据操作语言。INSERT \ DELETE \ UPDATE \ SELECT （重中之重）
DCL:数据控制语言。COMMIT \ ROLLBACK \ SAVEPOINT \ GRANT \ REVOKE

2.1 SQL的规则 ----必须要遵守

SQL 可以写在一行或者多行。为了提高可读性，各子句分行写，必要时使用缩进
每条命令以 ; 或 \g 或 \G 结束
关键字不能被缩写也不能分行
关于标点符号

必须保证所有的()、单引号、双引号是成对结束的
必须使用英文状态下的半角输入方式
字符串型和日期时间类型的数据可以使用单引号（’ '）表示
列的别名，尽量使用双引号（" "），而且不建议省略as

2.2 SQL的规范 ----建议遵守

MySQL 在 Windows 环境下是大小写不敏感的
MySQL 在 Linux 环境下是大小写敏感的

数据库名、表名、表的别名、变量名是严格区分大小写的
关键字、函数名、列名(或字段名)、列的别名(字段的别名) 是忽略大小写的。

推荐采用统一的书写规范：

数据库名、表名、表别名、字段名、字段别名等都小写
SQL 关键字、函数名、绑定变量等都大写

3. MySQL的三种注释的方式

USE dbtest2;

-- 这是一个查询语句
SELECT * FROM emp;

INSERT INTO emp 
VALUES(1002,'Tom'); #字符串、日期时间类型的变量需要使用一对''表示

INSERT INTO emp 
VALUES(1003,'Jerry');

# SELECT * FROM emp\G
SHOW CREATE TABLE emp\g

4. 导入现有的数据表、表的数据。

方式1：source 文件的全路径名
举例：source d:\st_db.sql;

方式2：基于具体的图形化界面的工具可以导入数据
比如：SQLyog中选择 “工具” – “执行sql脚本” – 选中xxx.sql即可。

5. 最基本的SELECT语句： SELECT 字段1,字段2,… FROM 表名

SELECT 1 + 1,3 * 2;

SELECT 1 + 1,3 * 2
FROM DUAL; #dual：伪表

*:表中的所有的字段（或列）

SELECT employee_id,last_name,salary
FROM employees;

6. 列的别名

as:全称：alias(别名),可以省略
列的别名可以使用一对""引起来，不要使用’'。

SELECT employee_id emp_id,last_name AS lname,department_id "部门id",salary * 12 AS "annual sal"
FROM employees;

7. 去除重复行

#查询员工表中一共有哪些部门id呢？
#错误的:没有去重的情况
SELECT department_id
FROM employees;
#正确的：去重的情况
SELECT DISTINCT department_id
FROM employees;

#错误的：
SELECT salary,DISTINCT department_id
FROM employees;

#仅仅是没有报错，但是没有实际意义。
SELECT DISTINCT department_id,salary
FROM employees;

8. 空值参与运算

空值：null
null不等同于0，‘’，‘null’

SELECT * FROM employees;

空值参与运算：结果一定也为空。

SELECT employee_id,salary "月工资",salary * (1 + commission_pct) * 12 "年工资",commission_pct
FROM employees;
#实际问题的解决方案：引入IFNULL
SELECT employee_id,salary "月工资",salary * (1 + IFNULL(commission_pct,0)) * 12 "年工资",commission_pct
FROM `employees`;

9. 着重号 ``

SELECT * FROM `order`;

10. 查询常数

SELECT '软件',123,employee_id,last_name
FROM employees;

11.显示表结构

DESCRIBE employees; #显示了表中字段的详细信息
DESC employees;
DESC departments;

12.过滤数据

#练习：查询90号部门的员工信息
SELECT * 
FROM employees
#过滤条件,声明在FROM结构的后面
WHERE department_id = 90;

#练习：查询last_name为'King'的员工信息
SELECT * 
FROM EMPLOYEES
WHERE LAST_NAME = 'King';

课后练习

# 1.查询员工12个月的工资总和，并起别名为ANNUAL SALARY
#理解1：计算12月的基本工资
SELECT employee_id,last_name,salary * 12 "ANNUAL SALARY"
FROM employees;

#理解2：计算12月的基本工资和奖金
SELECT employee_id,last_name,salary * 12 * (1 + IFNULL(commission_pct,0)) "ANNUAL SALARY"
FROM employees;


# 2.查询employees表中去除重复的job_id以后的数据
SELECT DISTINCT job_id
FROM employees;

# 3.查询工资大于12000的员工姓名和工资
SELECT last_name,salary
FROM employees
WHERE salary > 12000;

# 4.查询员工号为176的员工的姓名和部门号
SELECT last_name,department_id
FROM employees
WHERE employee_id = 176;

# 5.显示表 departments 的结构，并查询其中的全部数据 
DESCRIBE departments;

SELECT * FROM departments;

2、运算符

1. 算术运算符： + - * / div % mod

SELECT 100, 100 + 0, 100 - 0, 100 + 50, 100 + 50 * 30, 100 + 35.5, 100 - 35.5 
FROM DUAL;

# 在SQL中，+没有连接的作用，就表示加法运算。此时，会将字符串转换为数值（隐式转换）
SELECT 100 + '1'  # 在Java语言中，结果是：1001。 
FROM DUAL;

SELECT 100 + 'a' #此时将'a'看做0处理
FROM DUAL;

SELECT 100 + NULL  # null值参与运算，结果为null
FROM DUAL;

SELECT 100, 100 * 1, 100 * 1.0, 100 / 1.0, 100 / 2,
100 + 2 * 5 / 2,100 / 3, 100 DIV 0  # 分母如果为0，则结果为null
FROM DUAL;

# 取模运算： % mod
SELECT 12 % 3,12 % 5, 12 MOD -5,-12 % 5,-12 % -5
FROM DUAL;

#练习：查询员工id为偶数的员工信息
SELECT employee_id,last_name,salary
FROM employees
WHERE employee_id % 2 = 0;

2. 比较运算符

2.1 = <=> <> != < <= > >=

# = 的使用
SELECT 1 = 2,1 != 2,1 = '1',1 = 'a',0 = 'a' #字符串存在隐式转换。如果转换数值不成功，则看做0
FROM DUAL;

SELECT 'a' = 'a','ab' = 'ab','a' = 'b' #两边都是字符串的话，则按照ANSI的比较规则进行比较。
FROM DUAL;

SELECT 1 = NULL,NULL = NULL # 只要有null参与判断，结果就为null
FROM DUAL;

SELECT last_name,salary,commission_pct
FROM employees
#where salary = 6000;
WHERE commission_pct = NULL;  #此时执行，不会有任何的结果

# <=> ：安全等于。 记忆技巧：为NULL而生。

SELECT 1 <=> 2,1 <=> '1',1 <=> 'a',0 <=> 'a'
FROM DUAL;

SELECT 1 <=> NULL, NULL <=> NULL
FROM DUAL;

#练习：查询表中commission_pct为null的数据有哪些
SELECT last_name,salary,commission_pct
FROM employees
WHERE commission_pct <=> NULL;

SELECT 3 <> 2,'4' <> NULL, '' != NULL,NULL != NULL
FROM DUAL;

2.2

#① IS NULL \ IS NOT NULL \ ISNULL
#练习：查询表中commission_pct为null的数据有哪些
SELECT last_name,salary,commission_pct
FROM employees
WHERE commission_pct IS NULL;
#或
SELECT last_name,salary,commission_pct
FROM employees
WHERE ISNULL(commission_pct);

#练习：查询表中commission_pct不为null的数据有哪些
SELECT last_name,salary,commission_pct
FROM employees
WHERE commission_pct IS NOT NULL;
#或
SELECT last_name,salary,commission_pct
FROM employees
WHERE NOT commission_pct <=> NULL;

#② LEAST() \ GREATEST 

SELECT LEAST('g','b','t','m'),GREATEST('g','b','t','m')
FROM DUAL;

SELECT LEAST(first_name,last_name),LEAST(LENGTH(first_name),LENGTH(last_name))
FROM employees;

#③ BETWEEN 条件下界1 AND 条件上界2  （查询条件1和条件2范围内的数据，包含边界）
#查询工资在6000 到 8000的员工信息
SELECT employee_id,last_name,salary
FROM employees
#where salary between 6000 and 8000;
WHERE salary >= 6000 && salary <= 8000;

#交换6000 和 8000之后，查询不到数据
SELECT employee_id,last_name,salary
FROM employees
WHERE salary BETWEEN 8000 AND 6000;

#查询工资不在6000 到 8000的员工信息
SELECT employee_id,last_name,salary
FROM employees
WHERE salary NOT BETWEEN 6000 AND 8000;
#where salary < 6000 or salary > 8000;

#④ in (set)\ not in (set)

#练习：查询部门为10,20,30部门的员工信息
SELECT last_name,salary,department_id
FROM employees
#where department_id = 10 or department_id = 20 or department_id = 30;
WHERE department_id IN (10,20,30);

#练习：查询工资不是6000,7000,8000的员工信息
SELECT last_name,salary,department_id
FROM employees
WHERE salary NOT IN (6000,7000,8000);

#⑤ LIKE :模糊查询
# % : 代表不确定个数的字符 （0个，1个，或多个）

#练习：查询last_name中包含字符'a'的员工信息
SELECT last_name
FROM employees
WHERE last_name LIKE '%a%';

#练习：查询last_name中以字符'a'开头的员工信息
SELECT last_name
FROM employees
WHERE last_name LIKE 'a%';

#练习：查询last_name中包含字符'a'且包含字符'e'的员工信息
#写法1：
SELECT last_name
FROM employees
WHERE last_name LIKE '%a%' AND last_name LIKE '%e%';
#写法2：
SELECT last_name
FROM employees
WHERE last_name LIKE '%a%e%' OR last_name LIKE '%e%a%';

# _ ：代表一个不确定的字符

#练习：查询第3个字符是'a'的员工信息
SELECT last_name
FROM employees
WHERE last_name LIKE '__a%';

#练习：查询第2个字符是_且第3个字符是'a'的员工信息
#需要使用转义字符: \ 
SELECT last_name
FROM employees
WHERE last_name LIKE '_\_a%';

#或者  (了解)
SELECT last_name
FROM employees
WHERE last_name LIKE '_$_a%' ESCAPE '$';

#⑥ REGEXP \ RLIKE :正则表达式

SELECT 'shkstart' REGEXP '^shk', 'shkstart' REGEXP 't$', 'shkstart' REGEXP 'hk'
FROM DUAL;

SELECT 'atguigu' REGEXP 'gu.gu','atguigu' REGEXP '[ab]'
FROM DUAL;

3. 逻辑运算符： OR || AND && NOT ! XOR

# or  and 
SELECT last_name,salary,department_id
FROM employees
#where department_id = 10 or department_id = 20;
#where department_id = 10 and department_id = 20;
WHERE department_id = 50 AND salary > 6000;

# not 
SELECT last_name,salary,department_id
FROM employees
#where salary not between 6000 and 8000;
#where commission_pct is not null;
WHERE NOT commission_pct <=> NULL;

# XOR :追求的"异"
SELECT last_name,salary,department_id
FROM employees
WHERE department_id = 50 XOR salary > 6000;

#注意：AND的优先级高于OR

4. 位运算符： & | ^ ~ >> <<

SELECT 12 & 5, 12 | 5,12 ^ 5 
FROM DUAL;

SELECT 10 & ~1 FROM DUAL;

#在一定范围内满足：每向左移动1位，相当于乘以2；每向右移动一位，相当于除以2。
SELECT 4 << 1 , 8 >> 1
FROM DUAL;

课后练习

# 1.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 not between 5000 and 12000;
WHERE salary < 5000 OR salary > 12000;

# 2.选择在20或50号部门工作的员工姓名和部门号
SELECT last_name,department_id
FROM employees
# where department_id in (20,50);
WHERE department_id = 20 OR department_id = 50;

# 3.选择公司中没有管理者的员工姓名及job_id

SELECT last_name,job_id,manager_id
FROM employees
WHERE manager_id IS NULL;

SELECT last_name,job_id,manager_id
FROM employees
WHERE manager_id <=> NULL;

# 4.选择公司中有奖金的员工姓名，工资和奖金级别
SELECT last_name,salary,commission_pct
FROM employees
WHERE commission_pct IS NOT NULL;


SELECT last_name,salary,commission_pct
FROM employees
WHERE NOT commission_pct <=> NULL;


# 5.选择员工姓名的第三个字母是a的员工姓名

SELECT last_name
FROM employees
WHERE last_name LIKE '__a%';


# 6.选择姓名中有字母a和k的员工姓名

SELECT last_name
FROM employees
WHERE last_name LIKE '%a%k%' OR last_name LIKE '%k%a%';
#where last_name like '%a%' and last_name LIKE '%k%';

# 7.显示出表 employees 表中 first_name 以 'e'结尾的员工信息

SELECT first_name,last_name
FROM employees
WHERE first_name LIKE '%e';

SELECT first_name,last_name
FROM employees
WHERE first_name REGEXP 'e$'; # 以e开头的写法：'^e'

# 8.显示出表 employees 部门编号在 80-100 之间的姓名、工种
SELECT last_name,job_id
FROM employees
#方式1：推荐
WHERE department_id BETWEEN 80 AND 100;
#方式2：推荐，与方式1相同
#where department_id >= 80 and department_id <= 100;
#方式3：仅适用于本题的方式。
#where department_id in (80,90,100);

SELECT * FROM departments;

# 9.显示出表 employees 的 manager_id 是 100,101,110 的员工姓名、工资、管理者id

SELECT last_name,salary,manager_id
FROM employees
WHERE manager_id IN (100,101,110);

3、排序与分页

1. 排序

# 如果没有使用排序操作，默认情况下查询返回的数据是按照添加数据的顺序显示的。
SELECT * FROM employees;

1.1 基本使用

# 使用 ORDER BY 对查询到的数据进行排序操作。
# 升序：ASC (ascend)
# 降序：DESC (descend)

# 练习：按照salary从高到低的顺序显示员工信息
SELECT employee_id,last_name,salary
FROM employees
ORDER BY salary DESC;

# 练习：按照salary从低到高的顺序显示员工信息
SELECT employee_id,last_name,salary
FROM employees
ORDER BY salary ASC;

SELECT employee_id,last_name,salary
FROM employees
ORDER BY salary; # 如果在ORDER BY 后没有显式指名排序的方式的话，则默认按照升序排列。

1.2 我们可以使用列的别名，进行排序

SELECT employee_id,salary,salary * 12 annual_sal
FROM employees
ORDER BY annual_sal;

#列的别名只能在 ORDER BY 中使用，不能在WHERE中使用。
#如下操作报错！
SELECT employee_id,salary,salary * 12 annual_sal
FROM employees
WHERE annual_sal > 81600;

1.3 强调格式：WHERE 需要声明在FROM后，ORDER BY之前。

SELECT employee_id,salary
FROM employees
WHERE department_id IN (50,60,70)
ORDER BY department_id DESC;

1.4 二级排序

#练习：显示员工信息，按照department_id的降序排列，salary的升序排列
SELECT employee_id,salary,department_id
FROM employees
ORDER BY department_id DESC,salary ASC;

2. 分页

2.1 mysql使用limit实现数据的分页显示

# 需求1：每页显示20条记录，此时显示第1页
SELECT employee_id,last_name
FROM employees
LIMIT 0,20;


# 需求2：每页显示20条记录，此时显示第2页
SELECT employee_id,last_name
FROM employees
LIMIT 20,20;


# 需求3：每页显示20条记录，此时显示第3页
SELECT employee_id,last_name
FROM employees
LIMIT 40,20;

#需求：每页显示pageSize条记录，此时显示第pageNo页：
#公式：LIMIT (pageNo-1) * pageSize,pageSize;

2.2 WHERE … ORDER BY …LIMIT 声明顺序如下：

# LIMIT的格式： 严格来说：LIMIT 位置偏移量,条目数
# 结构"LIMIT 0,条目数" 等价于 "LIMIT 条目数"

SELECT employee_id,last_name,salary
FROM employees
WHERE salary > 6000
ORDER BY salary DESC
#limit 0,10;
LIMIT 10;

#练习：表里有107条数据，我们只想要显示第 32、33 条数据怎么办呢？

SELECT employee_id,last_name
FROM employees
LIMIT 31,2;

2.3 MySQL8.0新特性：LIMIT … OFFSET …

#练习：表里有107条数据，我们只想要显示第 32、33 条数据怎么办呢？

SELECT employee_id,last_name
FROM employees
LIMIT 2 OFFSET 31;

#练习：查询员工表中工资最高的员工信息
SELECT employee_id,last_name,salary
FROM employees
ORDER BY salary DESC
#limit 0,1
LIMIT 1;

2.4 LIMIT 可以使用在MySQL、PGSQL、MariaDB、SQLite 等数据库中使用，表示分页。

不能使用在SQL Server、DB2、Oracle！

课后练习

#1. 查询员工的姓名和部门号和年薪，按年薪降序,按姓名升序显示 

SELECT last_name,department_id,salary * 12 annual_salary
FROM employees
ORDER BY annual_salary DESC,last_name ASC;

#2. 选择工资不在 8000 到 17000 的员工的姓名和工资，按工资降序，显示第21到40位置的数据 

SELECT last_name,salary
FROM employees
WHERE salary NOT BETWEEN 8000 AND 17000
ORDER BY salary DESC
LIMIT 20,20;


#3. 查询邮箱中包含 e 的员工信息，并先按邮箱的字节数降序，再按部门号升序

SELECT employee_id,last_name,email,department_id
FROM employees
#where email like '%e%'
WHERE email REGEXP '[e]'
ORDER BY LENGTH(email) DESC,department_id;

4、多表查询

SELECT ...,....,....
FROM ....
WHERE .... AND / OR / NOT....
ORDER BY .... (ASC/DESC),....,...
LIMIT ...,...

1. 熟悉常见的几个表

DESC employees;

DESC departments;

DESC locations;

#查询员工名为'Abel'的人在哪个城市工作？
SELECT * 
FROM employees
WHERE last_name = 'Abel';

SELECT *
FROM departments
WHERE department_id = 80;


SELECT *
FROM locations 
WHERE location_id = 2500;

2. 出现笛卡尔积的错误

#错误的原因：缺少了多表的连接条件

#错误的实现方式：每个员工都与每个部门匹配了一遍。
SELECT employee_id,department_name
FROM employees,departments;  #查询出2889条记录

#错误的方式
SELECT employee_id,department_name
FROM employees CROSS JOIN departments;#查询出2889条记录


SELECT *
FROM employees;  #107条记录

SELECT 2889 / 107
FROM DUAL;

SELECT *
FROM departments; # 27条记录

3. 多表查询的正确方式：需要有连接条件

SELECT employee_id,department_name
FROM employees,departments
#两个表的连接条件
WHERE employees.`department_id` = departments.department_id;

4. 如果查询语句中出现了多个表中都存在的字段，则必须指明此字段所在的表。

SELECT employees.employee_id,departments.department_name,employees.department_id
FROM employees,departments
WHERE employees.`department_id` = departments.department_id;

#建议：从sql优化的角度，建议多表查询时，每个字段前都指明其所在的表。

5. 可以给表起别名，在SELECT和WHERE中使用表的别名。

SELECT emp.employee_id,dept.department_name,emp.department_id
FROM employees emp,departments dept
WHERE emp.`department_id` = dept.department_id;

#如果给表起了别名，一旦在SELECT或WHERE中使用表名的话，则必须使用表的别名，而不能再使用表的原名。
#如下的操作是错误的：
SELECT emp.employee_id,departments.department_name,emp.department_id
FROM employees emp,departments dept
WHERE emp.`department_id` = departments.department_id;

6. 结论：如果有n个表实现多表的查询，则需要至少n-1个连接条件

#练习：查询员工的employee_id,last_name,department_name,city
SELECT e.employee_id,e.last_name,d.department_name,l.city,e.department_id,l.location_id
FROM employees e,departments d,locations l
WHERE e.`department_id` = d.`department_id`
AND d.`location_id` = l.`location_id`;

/*
演绎式：提出问题1 ---> 解决问题1 ----> 提出问题2 ---> 解决问题2 ....

归纳式：总--分

*/

7. 多表查询的分类

角度1：等值连接  vs  非等值连接

角度2：自连接  vs  非自连接

角度3：内连接  vs  外连接

7.1 等值连接 vs 非等值连接

#非等值连接的例子：
SELECT *
FROM job_grades;

SELECT e.last_name,e.salary,j.grade_level
FROM employees e,job_grades j
#where e.`salary` between j.`lowest_sal` and j.`highest_sal`;
WHERE e.`salary` >= j.`lowest_sal` AND e.`salary` <= j.`highest_sal`;

7.2 自连接 vs 非自连接

SELECT * FROM employees;

#自连接的例子：
#练习：查询员工id,员工姓名及其管理者的id和姓名

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`;

7.3 内连接 vs 外连接

# 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
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id;  #只有106条记录

# 外连接：合并具有同一列的两个以上的表的行, 结果集中除了包含一个表与另一个表匹配的行之外，
#         还查询到了左表 或 右表中不匹配的行。

# 外连接的分类：左外连接、右外连接、满外连接

# 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。

#练习：查询所有的员工的last_name,department_name信息 

SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id;   # 需要使用左外连接

#SQL92语法实现内连接：见上，略
#SQL92语法实现外连接：使用 +  ----------MySQL不支持SQL92语法中外连接的写法！
#不支持：
SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id(+);

#SQL99语法中使用 JOIN ...ON 的方式实现多表的查询。这种方式也能解决外连接的问题。MySQL是支持此种方式的。
#SQL99语法如何实现多表的查询。

#SQL99语法实现内连接：
SELECT last_name,department_name
FROM employees e INNER JOIN departments d
ON e.`department_id` = d.`department_id`;

SELECT 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`;

#SQL99语法实现外连接：

#练习：查询所有的员工的last_name,department_name信息 
# 左外连接：
SELECT last_name,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`;

#右外连接：
SELECT last_name,department_name
FROM employees e RIGHT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;


#满外连接：mysql不支持FULL OUTER JOIN
SELECT last_name,department_name
FROM employees e FULL OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;

8. UNION 和 UNION ALL的使用

# UNION：会执行去重操作
# UNION ALL:不会执行去重操作
#结论：如果明确知道合并数据后的结果数据不存在重复数据，或者不需要去除重复的数据，
#则尽量使用UNION ALL语句，以提高数据查询的效率。

9. 7种JOIN的实现：

# 中图：内连接
SELECT employee_id,department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`;

# 左上图：左外连接
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`;

# 右上图：右外连接
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`;

# 左中图：
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL;

# 右中图：
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;


# 左下图：满外连接
# 方式1：左上图 UNION ALL 右中图

SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;


# 方式2：左中图 UNION ALL 右上图

SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`;

# 右下图：左中图  UNION ALL 右中图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL;

10. SQL99语法的新特性1:自然连接

SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`
AND e.`manager_id` = d.`manager_id`;

# NATURAL JOIN : 它会帮你自动查询两张连接表中`所有相同的字段`，然后进行`等值连接`。
SELECT employee_id,last_name,department_name
FROM employees e NATURAL JOIN departments d;

11. SQL99语法的新特性2:USING

SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
ON e.department_id = d.department_id;

SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
USING (department_id);


#拓展：
SELECT last_name,job_title,department_name 
FROM employees INNER JOIN departments INNER JOIN jobs 
ON employees.department_id = departments.department_id 
AND employees.job_id = jobs.job_id;

课后练习

# 1.显示所有员工的姓名，部门号和部门名称。
SELECT e.last_name,e.department_id,d.department_name
FROM employees e LEFT OUTER JOIN departments d
ON e.`department_id` = d.`department_id`;

# 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 d.`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 ,e.`commission_pct`, 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; #也应该是35条记录

SELECT *
FROM employees
WHERE commission_pct IS NOT NULL; #35条记录

# 4.选择city在Toronto工作的员工的 last_name , job_id , department_id , department_name 

SELECT e.last_name , e.job_id , e.department_id , d.department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`
JOIN locations l
ON d.`location_id` = l.`location_id`
WHERE l.`city` = 'Toronto';

#sql92语法：
SELECT e.last_name , e.job_id , e.department_id , d.department_name
FROM employees e,departments d ,locations l
WHERE e.`department_id` = d.`department_id` 
AND d.`location_id` = l.`location_id`
AND l.`city` = 'Toronto';

# 5.查询员工所在的部门名称、部门地址、姓名、工作、工资，其中员工所在部门的部门名称为’Executive’

SELECT d.department_name,l.street_address,e.last_name,e.job_id,e.salary
FROM departments d LEFT JOIN employees e
ON e.`department_id` = d.`department_id`
LEFT JOIN locations l
ON d.`location_id` = l.`location_id`
WHERE d.`department_name` = 'Executive';

# 6.选择指定员工的姓名，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	100

SELECT 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;

# 7.查询哪些部门没有员工
SELECT d.department_id
FROM departments d LEFT JOIN employees e
ON d.`department_id` = e.`department_id`
WHERE e.`department_id` IS NULL;

# 8. 查询哪个城市没有部门 

SELECT l.location_id,l.city
FROM locations l LEFT JOIN departments d
ON l.`location_id` = d.`location_id`
WHERE d.`location_id` IS NULL;

SELECT department_id
FROM departments
WHERE department_id IN (1000,1100,1200,1300,1600);
# 9. 查询部门名为 Sales 或 IT 的员工信息

SELECT e.employee_id,e.last_name,e.department_id
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_name` IN ('Sales','IT');