From e49865b4336bf6e411d0a5174ecb309dc2a60494 Mon Sep 17 00:00:00 2001 From: wxin <15253413025@163.com> Date: Tue, 11 Mar 2025 18:54:38 +0800 Subject: [PATCH] =?UTF-8?q?=E4=B8=8A=E4=BC=A0=E6=96=87=E4=BB=B6=E8=87=B3?= =?UTF-8?q?=20/?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- 数据库查询.md | 965 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 965 insertions(+) create mode 100644 数据库查询.md diff --git a/数据库查询.md b/数据库查询.md new file mode 100644 index 0000000..6dff674 --- /dev/null +++ b/数据库查询.md @@ -0,0 +1,965 @@ +

数据库查询

+ +------ + +## 一：基本查询 + +### 1. 简介 + +- 单表查询 +- 简单查询 +- 通过条件查询 +- 查询排序 +- 限制查询记录数 +- 使用集合函数查询 +- 分组查询 +- 使用正则表达式查询 + +### 2. 案例 + +创建案例所需表：company.employee1 + +``` + 雇员编号 id int + 雇员姓名 name varchar(30) + 雇员性别 sex enum + 雇用时期 hire_date date + 职位 post varchar(50) + 职位描述 job_description varchar(100) + 薪水 salary double(15,2) + 办公室 office int + 部门编号 dep_id int +``` + +```SQL +mysql> create table company.employee1( + -> id int primary key auto_increment not null, + -> name varchar(30) not null, + -> sex enum('m','f') default 'm' not null, + -> hire_date date not null, + -> hire_date varchar(50) not null, + -> job_description varchar(100), + -> salary double(15,2) not null, + -> office int, + -> dep_id int + -> ); +Query OK, 0 rows affected (0.00 sec) +``` + +插入模拟数据： + +```SQL +mysql> insert into company.employee1(name,sex,hire_date,post,job_description,salary,office,dep_id) values + -> ('jack','m','20180202','instructor','teach',5000,501,100), + -> ('tom','m','20180203','instructor','teach',5500,501,100), + -> ('robin','m','20180202','instructor','teach',8000,501,100), + -> ('alice','f','20180202','instructor','teach',7200,501,100), + -> ('','m','20180202','hr','hrcc',600,502,101), + -> ('harry','m','20180202','hr',NULL,6000,502,101), + -> ('emma','f','20180206','sale','salecc',20000,503,102), + -> ('christine','f','20180205','sale','salecc',2200,503,102), + -> ('zhuzhu','m','20180205','sale',NULL,2200,503,102), + -> ('gougou','m','20180205','sale','',2200,503,102); +Query OK, 10 rows affected (0.00 sec) +Records: 10 Duplicates: 0 Warnings: 0 +``` + +语法格式： + +​ select 字段名称,字段名称2...... from 表名 [条件] + +#### 简单查询 + +```SQL +mysql> select * from employee1; ++----+-----------+-----+------------+-----------------+----------+--------+--------+------------+ +| id | name | sex | hire_date | job_description | salary | office | dep_id | post | ++----+-----------+-----+------------+-----------------+----------+--------+--------+------------+ +| 1 | jack | m | 2018-02-02 | teach | 5000.00 | 501 | 100 | instructor | +| 2 | tom | m | 2018-02-03 | teach | 5500.00 | 501 | 100 | instructor | +| 3 | robin | m | 2018-02-02 | teach | 8000.00 | 501 | 100 | instructor | +| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor | +| 5 | | m | 2018-02-02 | hrcc | 600.00 | 502 | 101 | hr | +| 6 | harry | m | 2018-02-02 | NULL | 6000.00 | 502 | 101 | hr | +| 7 | emma | f | 2018-02-06 | salecc | 20000.00 | 503 | 102 | sale | +| 8 | christine | f | 2018-02-05 | salecc | 2200.00 | 503 | 102 | sale | +| 9 | zhuzhu | m | 2018-02-05 | NULL | 2200.00 | 503 | 102 | sale | +| 10 | gougou | m | 2018-02-05 | | 2200.00 | 503 | 102 | sale | ++----+-----------+-----+------------+-----------------+----------+--------+--------+------------+ +10 rows in set (0.00 sec) + +mysql> select name,salary,dep_id from employee1 where id <= 5; ++-------+---------+--------+ +| name | salary | dep_id | ++-------+---------+--------+ +| jack | 5000.00 | 100 | +| tom | 5500.00 | 100 | +| robin | 8000.00 | 100 | +| alice | 7200.00 | 100 | +| | 600.00 | 101 | ++-------+---------+--------+ +5 rows in set (0.00 sec) +``` + +#### 避免重复 + +不能部分使用DISTINCT，通常仅用于某一字段 + +```SQL +mysql> select post from employee1; ++------------+ +| post | ++------------+ +| instructor | +| instructor | +| instructor | +| instructor | +| hr | +| hr | +| sale | +| sale | +| sale | +| sale | ++------------+ +10 rows in set (0.00 sec) + +mysql> select distinct post from employee1; ++------------+ +| post | ++------------+ +| instructor | +| hr | +| sale | ++------------+ +3 rows in set (0.00 sec) +``` + +#### 四则运算查询 + +```SQL +mysql> select name,salary,salary*14 from employee1; ++-----------+----------+-----------+ +| name | salary | salary*14 | ++-----------+----------+-----------+ +| jack | 5000.00 | 70000.00 | +| tom | 5500.00 | 77000.00 | +| robin | 8000.00 | 112000.00 | +| alice | 7200.00 | 100800.00 | +| | 600.00 | 8400.00 | +| harry | 6000.00 | 84000.00 | +| emma | 20000.00 | 280000.00 | +| christine | 2200.00 | 30800.00 | +| zhuzhu | 2200.00 | 30800.00 | +| gougou | 2200.00 | 30800.00 | ++-----------+----------+-----------+ +10 rows in set (0.00 sec) + +mysql> select name,salary,salary*14 as Annual_salary from employee1; ++-----------+----------+---------------+ +| name | salary | Annual_salary | ++-----------+----------+---------------+ +| jack | 5000.00 | 70000.00 | +| tom | 5500.00 | 77000.00 | +| robin | 8000.00 | 112000.00 | +| alice | 7200.00 | 100800.00 | +| | 600.00 | 8400.00 | +| harry | 6000.00 | 84000.00 | +| emma | 20000.00 | 280000.00 | +| christine | 2200.00 | 30800.00 | +| zhuzhu | 2200.00 | 30800.00 | +| gougou | 2200.00 | 30800.00 | ++-----------+----------+---------------+ +10 rows in set (0.00 sec) + +mysql> select name,salary,salary*14 Annual_salary from employee1; ++-----------+----------+---------------+ +| name | salary | Annual_salary | ++-----------+----------+---------------+ +| jack | 5000.00 | 70000.00 | +| tom | 5500.00 | 77000.00 | +| robin | 8000.00 | 112000.00 | +| alice | 7200.00 | 100800.00 | +| | 600.00 | 8400.00 | +| harry | 6000.00 | 84000.00 | +| emma | 20000.00 | 280000.00 | +| christine | 2200.00 | 30800.00 | +| zhuzhu | 2200.00 | 30800.00 | +| gougou | 2200.00 | 30800.00 | ++-----------+----------+---------------+ +10 rows in set (0.00 sec) +``` + +#### 定义显示格式 + +CONCAT() 函数用于连接字符串 + +```SQL +mysql> select concat(name,'s annual salary:',salary*14) as Annual_salary from employee1; ++-----------------------------------+ +| Annual_salary | ++-----------------------------------+ +| jacks annual salary:70000.00 | +| toms annual salary:77000.00 | +| robins annual salary:112000.00 | +| alices annual salary:100800.00 | +| s annual salary:8400.00 | +| harrys annual salary:84000.00 | +| emmas annual salary:280000.00 | +| christines annual salary:30800.00 | +| zhuzhus annual salary:30800.00 | +| gougous annual salary:30800.00 | ++-----------------------------------+ +10 rows in set (0.00 sec) +``` + +#### 单条件查询 + +```SQL +mysql> select name,post from employee1 where post='hr'; ++-------+------+ +| name | post | ++-------+------+ +| | hr | +| harry | hr | ++-------+------+ +2 rows in set (0.00 sec) +``` + +#### 多条件查询 + +```SQL +mysql> select name,salary from employee1 where post='hr' and salary>10000; +Empty set (0.00 sec) + +mysql> select * from employee1 where salary>5000 and salary<10000 or dep_id=102; ++----+-----------+-----+------------+-----------------+----------+--------+--------+------------+ +| id | name | sex | hire_date | job_description | salary | office | dep_id | post | ++----+-----------+-----+------------+-----------------+----------+--------+--------+------------+ +| 2 | tom | m | 2018-02-03 | teach | 5500.00 | 501 | 100 | instructor | +| 3 | robin | m | 2018-02-02 | teach | 8000.00 | 501 | 100 | instructor | +| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor | +| 6 | harry | m | 2018-02-02 | NULL | 6000.00 | 502 | 101 | hr | +| 7 | emma | f | 2018-02-06 | salecc | 20000.00 | 503 | 102 | sale | +| 8 | christine | f | 2018-02-05 | salecc | 2200.00 | 503 | 102 | sale | +| 9 | zhuzhu | m | 2018-02-05 | NULL | 2200.00 | 503 | 102 | sale | +| 10 | gougou | m | 2018-02-05 | | 2200.00 | 503 | 102 | sale | ++----+-----------+-----+------------+-----------------+----------+--------+--------+------------+ +8 rows in set (0.00 sec) +``` + +#### 关键字 + +BETWEEN AND + +```SQL +mysql> select name,salary from employee1 where salary between 5000 and 15000; ++-------+---------+ +| name | salary | ++-------+---------+ +| jack | 5000.00 | +| tom | 5500.00 | +| robin | 8000.00 | +| alice | 7200.00 | +| harry | 6000.00 | ++-------+---------+ +5 rows in set (0.00 sec) + +mysql> select name,salary from employee1 where salary not between 5000 and 15000; ++-----------+----------+ +| name | salary | ++-----------+----------+ +| | 600.00 | +| emma | 20000.00 | +| christine | 2200.00 | +| zhuzhu | 2200.00 | +| gougou | 2200.00 | ++-----------+----------+ +5 rows in set (0.00 sec) +``` + +IS NULL + +```SQL +mysql> select name,job_description from employee1 where job_description is null; ++--------+-----------------+ +| name | job_description | ++--------+-----------------+ +| harry | NULL | +| zhuzhu | NULL | ++--------+-----------------+ +2 rows in set (0.00 sec) + +mysql> select name,job_description from employee1 where job_description is not null; ++-----------+-----------------+ +| name | job_description | ++-----------+-----------------+ +| jack | teach | +| tom | teach | +| robin | teach | +| alice | teach | +| | hrcc | +| emma | salecc | +| christine | salecc | +| gougou | | ++-----------+-----------------+ +8 rows in set (0.00 sec) + +mysql> select name,job_description from employee1 where job_description=''; ++--------+-----------------+ +| name | job_description | ++--------+-----------------+ +| gougou | | ++--------+-----------------+ +1 row in set (0.00 sec) +``` + +注意：（NULL说明） + +- 等价于没有任何值、是未知数 +- NULL与0、空字符串、空格都不同,NULL没有分配存储空间 +- 对空值做加、减、乘、除等运算操作，结果仍为空 +- 比较时使用关键字用“is null”和“is not null” +- 排序时比其他数据都小（索引默认是降序排列，小→大），所以NULL值总是排在最前 + +#### IN集合查询 + +```SQL +mysql> select name,salary from employee1 where salary=4000 or salary=5000 or salary=6000 or salary=9000; ++-------+---------+ +| name | salary | ++-------+---------+ +| jack | 5000.00 | +| harry | 6000.00 | ++-------+---------+ +2 rows in set (0.00 sec) + +mysql> select name,salary from employee1 where salary in (4000,5000,6000,9000); ++-------+---------+ +| name | salary | ++-------+---------+ +| jack | 5000.00 | +| harry | 6000.00 | ++-------+---------+ +2 rows in set (0.00 sec) + +mysql> select name,salary from employee1 where salary not in (4000,5000,6000,9000); ++-----------+----------+ +| name | salary | ++-----------+----------+ +| tom | 5500.00 | +| robin | 8000.00 | +| alice | 7200.00 | +| | 600.00 | +| emma | 20000.00 | +| christine | 2200.00 | +| zhuzhu | 2200.00 | +| gougou | 2200.00 | ++-----------+----------+ +8 rows in set (0.00 sec) +``` + +#### 模糊查询 + +关键字LIKE + +通配符%：所有字符 + +通配符_： 一个字符 + +```SQL +mysql> select * from employee1 where name like 'al%'; ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +| id | name | sex | hire_date | job_description | salary | office | dep_id | post | ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor | ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +1 row in set (0.00 sec) + +mysql> select * from employee1 where name like 'al___'; ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +| id | name | sex | hire_date | job_description | salary | office | dep_id | post | ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor | ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +1 row in set (0.00 sec) +``` + +#### 排序查询 + +```SQL +mysql> select name,salary from employee1 order by salary; //升序 ++-----------+----------+ +| name | salary | ++-----------+----------+ +| | 600.00 | +| christine | 2200.00 | +| zhuzhu | 2200.00 | +| gougou | 2200.00 | +| jack | 5000.00 | +| tom | 5500.00 | +| harry | 6000.00 | +| alice | 7200.00 | +| robin | 8000.00 | +| emma | 20000.00 | ++-----------+----------+ +10 rows in set (0.00 sec) + +mysql> select name,salary from employee1 order by salary desc; //降序 ++-----------+----------+ +| name | salary | ++-----------+----------+ +| emma | 20000.00 | +| robin | 8000.00 | +| alice | 7200.00 | +| harry | 6000.00 | +| tom | 5500.00 | +| jack | 5000.00 | +| christine | 2200.00 | +| zhuzhu | 2200.00 | +| gougou | 2200.00 | +| | 600.00 | ++-----------+----------+ +10 rows in set (0.00 sec) + +mysql> select name,salary from employee1 order by salary desc limit 3; //控制显示前3行 ++-------+----------+ +| name | salary | ++-------+----------+ +| emma | 20000.00 | +| robin | 8000.00 | +| alice | 7200.00 | ++-------+----------+ +3 rows in set (0.00 sec) + +mysql> select name,salary from employee1 order by salary desc limit 1,3; //从序号1开始显示三行的内容 ++-------+---------+ +| name | salary | ++-------+---------+ +| robin | 8000.00 | +| alice | 7200.00 | +| harry | 6000.00 | ++-------+---------+ +3 rows in set (0.00 sec) +``` + +注意： + +- ascending 美音 /ə'sɛndɪŋ/ 升序 +- descending 美音 /dɪ'sɛndɪŋ/ 降序 + +#### 集合函数查询 + +count：可以查看共有多少条记录 + +```SQL +mysql> select count(*) from employee1; ++----------+ +| count(*) | ++----------+ +| 10 | ++----------+ +1 row in set (0.00 sec) + +mysql> select count(name) from employee1; ++-------------+ +| count(name) | ++-------------+ +| 10 | ++-------------+ +1 row in set (0.01 sec) +``` + +max：查看最大值 + +```SQL +mysql> select max(salary) from employee1; ++-------------+ +| max(salary) | ++-------------+ +| 20000.00 | ++-------------+ +1 row in set (0.00 sec) +``` + +min：查看最小值 + +```SQL +mysql> select min(salary) from employee1; ++-------------+ +| min(salary) | ++-------------+ +| 600.00 | ++-------------+ +1 row in set (0.00 sec) +``` + +avg：查看平均值 + +```SQL +mysql> select avg(salary) from employee1; ++-------------+ +| avg(salary) | ++-------------+ +| 5890.000000 | ++-------------+ +1 row in set (0.00 sec) +``` + +sum：求和 + +sale这个部门的总工资 + +```SQL +mysql> select concat("Total Department Wages:",sum(salary)) from employee1 where post='sale'; ++-----------------------------------------------+ +| concat("Total Department Wages:",sum(salary)) | ++-----------------------------------------------+ +| Total Department Wages:26600.00 | ++-----------------------------------------------+ +1 row in set (0.00 sec) +``` + +获取薪水最高的这个人的详细信息 + +```SQL +mysql> select * from employee1 where salary=(select max(salary) from employee1); ++----+------+-----+------------+-----------------+----------+--------+--------+------+ +| id | name | sex | hire_date | job_description | salary | office | dep_id | post | ++----+------+-----+------------+-----------------+----------+--------+--------+------+ +| 7 | emma | f | 2018-02-06 | salecc | 20000.00 | 503 | 102 | sale | ++----+------+-----+------------+-----------------+----------+--------+--------+------+ +1 row in set (0.00 sec) +``` + +#### 分组查询 + +GROUP BY和GROUP_CONCAT()函数一起使用 + +获取部门ID相同的员工并把名字拼接到一起 + +```SQL +mysql> select dep_id,group_concat(name) from employee1 group by dep_id; ++--------+------------------------------+ +| dep_id | group_concat(name) | ++--------+------------------------------+ +| 100 | jack,tom,robin,alice | +| 101 | ,harry | +| 102 | emma,christine,zhuzhu,gougou | ++--------+------------------------------+ +3 rows in set (0.00 sec) +``` + +GROUP BY和集合函数一起使用 + +获取部门最高薪资 + +```SQL +mysql> select post,max(salary) from employee1 group by post; ++------------+-------------+ +| post | max(salary) | ++------------+-------------+ +| hr | 6000.00 | +| instructor | 8000.00 | +| sale | 20000.00 | ++------------+-------------+ +3 rows in set (0.00 sec) +``` + +#### 正则查询 + +```SQL +以什么开头 +mysql> select * from employee1 where name regexp '^ali'; ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +| id | name | sex | hire_date | job_description | salary | office | dep_id | post | ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor | ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +1 row in set (0.00 sec) + +以什么开头 +mysql> select * from employee1 where name regexp 'ce$'; ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +| id | name | sex | hire_date | job_description | salary | office | dep_id | post | ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +| 4 | alice | f | 2018-02-02 | teach | 7200.00 | 501 | 100 | instructor | ++----+-------+-----+------------+-----------------+---------+--------+--------+------------+ +1 row in set (0.00 sec) + +连续出现n次 +mysql> select * from employee1 where name regexp 'm{2}'; ++----+------+-----+------------+-----------------+----------+--------+--------+------+ +| id | name | sex | hire_date | job_description | salary | office | dep_id | post | ++----+------+-----+------------+-----------------+----------+--------+--------+------+ +| 7 | emma | f | 2018-02-06 | salecc | 20000.00 | 503 | 102 | sale | ++----+------+-----+------------+-----------------+----------+--------+--------+------+ +1 row in set (0.00 sec) +``` + + + +## 二：多表联合查询 + +### 1.数据准备 + +**表：company.employee2** + +创建表： + +```SQL +mysql> create table employee2( + -> emp_id int auto_increment primary key not null, + -> emp_name varchar(50), + -> age int, + -> dept_id int + -> ); +Query OK, 0 rows affected (0.00 sec) +``` + +查看表结构： + +```SQL +mysql> desc employee2; ++----------+-------------+------+-----+---------+----------------+ +| Field | Type | Null | Key | Default | Extra | ++----------+-------------+------+-----+---------+----------------+ +| emp_id | int(11) | NO | PRI | NULL | auto_increment | +| emp_name | varchar(50) | YES | | NULL | | +| age | int(11) | YES | | NULL | | +| dept_id | int(11) | YES | | NULL | | ++----------+-------------+------+-----+---------+----------------+ +4 rows in set (0.00 sec) +``` + +插入模拟数据： + +```SQL +mysql> insert into employee2(emp_name,age,dept_id) values + -> ('',19,200), + -> ('tom',26,201), + -> ('jack',30,201), + -> ('alice',24,202), + -> ('robin',40,200), + -> ('wxin',16,200), + -> ('natasha',28,204); +Query OK, 7 rows affected (0.00 sec) +Records: 7 Duplicates: 0 Warnings: 0 +``` + +查看数据： + +```SQL +mysql> select * from employee2; ++--------+----------+------+---------+ +| emp_id | emp_name | age | dept_id | ++--------+----------+------+---------+ +| 1 | | 19 | 200 | +| 2 | tom | 26 | 201 | +| 3 | jack | 30 | 201 | +| 4 | alice | 24 | 202 | +| 5 | robin | 40 | 200 | +| 6 | wxin | 16 | 200 | +| 7 | natasha | 28 | 204 | ++--------+----------+------+---------+ +7 rows in set (0.00 sec) +``` + +**表：company.department2** + +创建表： + +```SQL +mysql> create table department2( + -> dept_id int, + -> dept_name varchar(100) + -> ); +Query OK, 0 rows affected (0.01 sec) +``` + +查看表结构： + +```SQL +mysql> desc department2; ++-----------+--------------+------+-----+---------+-------+ +| Field | Type | Null | Key | Default | Extra | ++-----------+--------------+------+-----+---------+-------+ +| dept_id | int(11) | YES | | NULL | | +| dept_name | varchar(100) | YES | | NULL | | ++-----------+--------------+------+-----+---------+-------+ +2 rows in set (0.00 sec) +``` + +模拟插入数据： + +```SQL +mysql> insert into department2 values + -> (200,'hr'), + -> (201,'it'), + -> (202,'sale'), + -> (203,'fd'); +Query OK, 4 rows affected (0.00 sec) +Records: 4 Duplicates: 0 Warnings: 0 +``` + +查看数据： + +```SQL +sql> select * from department2; ++---------+-----------+ +| dept_id | dept_name | ++---------+-----------+ +| 200 | hr | +| 201 | it | +| 202 | sale | +| 203 | fd | ++---------+-----------+ +4 rows in set (0.00 sec) +``` + +### 2. 多表的连接查询 + +- 交叉连接：生成笛卡尔积，它不使用任何匹配条件；交叉联接返回左表中的所有行，左表中的每一行与右表中的所有行组合 +- 内连接：只连接匹配的行 +- 外连接： + 1. 左连接：会显示左边表内所有的值，不论在右边表内匹不匹配 + 2. 右连接：会显示右边表内所有的值，不论在左边表内匹不匹配 +- 全外连接：包含左、右两个表的全部行 + +**交叉连接** + +```SQL +mysql> select employee2.emp_name,employee2.age,employee2.dept_id,department2.dept_name from employee2,department2; ++----------+------+---------+-----------+ +| emp_name | age | dept_id | dept_name | ++----------+------+---------+-----------+ +| | 19 | 200 | hr | +| | 19 | 200 | it | +| | 19 | 200 | sale | +| | 19 | 200 | fd | +| tom | 26 | 201 | hr | +| tom | 26 | 201 | it | +| tom | 26 | 201 | sale | +| tom | 26 | 201 | fd | +| jack | 30 | 201 | hr | +| jack | 30 | 201 | it | +| jack | 30 | 201 | sale | +| jack | 30 | 201 | fd | +| alice | 24 | 202 | hr | +| alice | 24 | 202 | it | +| alice | 24 | 202 | sale | +| alice | 24 | 202 | fd | +| robin | 40 | 200 | hr | +| robin | 40 | 200 | it | +| robin | 40 | 200 | sale | +| robin | 40 | 200 | fd | +| wxin | 16 | 200 | hr | +| wxin | 16 | 200 | it | +| wxin | 16 | 200 | sale | +| wxin | 16 | 200 | fd | +| natasha | 28 | 204 | hr | +| natasha | 28 | 204 | it | +| natasha | 28 | 204 | sale | +| natasha | 28 | 204 | fd | ++----------+------+---------+-----------+ +28 rows in set (0.00 sec) +``` + +**内连接** + +获取有部门的员工 (部门表中没有natasha所在的部门) + +```SQL +mysql> select employee2.emp_name,employee2.age,employee2.dept_id,department2.dept_name from employee2,department2 where employee2.dept_id=department2.dept_id; ++----------+------+---------+-----------+ +| emp_name | age | dept_id | dept_name | ++----------+------+---------+-----------+ +| | 19 | 200 | hr | +| tom | 26 | 201 | it | +| jack | 30 | 201 | it | +| alice | 24 | 202 | sale | +| robin | 40 | 200 | hr | +| wxin | 16 | 200 | hr | ++----------+------+---------+-----------+ +6 rows in set (0.00 sec) + +mysql> select employee2.emp_name,department2.dept_name from employee2 inner join department2 on employee2.dept_id=department2.dept_id; ++----------+-----------+ +| emp_name | dept_name | ++----------+-----------+ +| | hr | +| tom | it | +| jack | it | +| alice | sale | +| robin | hr | +| wxin | hr | ++----------+-----------+ +6 rows in set (0.00 sec) +``` + +**外连接** + +语法： + +​ SELECT 字段列表 FROM 表1 LEFT|RIGHT JOIN 表2 ON 表1.字段 = 表2.字段; + +注意： + +​ 先用谁谁就是左 + +**左连接 left join** + +```SQL +找出所有员工及所属的部门，包括没有部门的员工 +mysql> select emp_id,emp_name,dept_name from employee2 left join department2 on employee2.dept_id=department2.dept_id; ++--------+----------+-----------+ +| emp_id | emp_name | dept_name | ++--------+----------+-----------+ +| 1 | | hr | +| 5 | robin | hr | +| 6 | wxin | hr | +| 2 | tom | it | +| 3 | jack | it | +| 4 | alice | sale | +| 7 | natasha | NULL | ++--------+----------+-----------+ +7 rows in set (0.00 sec) +``` + +**右连接 right join** + +```SQL +找出所有部门包含的员工，包括空部门 +mysql> select emp_id,emp_name,dept_name from employee2 right join department2 on employee2.dept_id=department2.dept_id; ++--------+----------+-----------+ +| emp_id | emp_name | dept_name | ++--------+----------+-----------+ +| 1 | | hr | +| 2 | tom | it | +| 3 | jack | it | +| 4 | alice | sale | +| 5 | robin | hr | +| 6 | wxin | hr | +| NULL | NULL | fd | ++--------+----------+-----------+ +7 rows in set (0.00 sec) +``` + +**全外连接** + +``` +mysql> select * from employee2 full join department2; ++--------+----------+------+---------+---------+-----------+ +| emp_id | emp_name | age | dept_id | dept_id | dept_name | ++--------+----------+------+---------+---------+-----------+ +| 1 | | 19 | 200 | 200 | hr | +| 1 | | 19 | 200 | 201 | it | +| 1 | | 19 | 200 | 202 | sale | +| 1 | | 19 | 200 | 203 | fd | +| 2 | tom | 26 | 201 | 200 | hr | +| 2 | tom | 26 | 201 | 201 | it | +| 2 | tom | 26 | 201 | 202 | sale | +| 2 | tom | 26 | 201 | 203 | fd | +| 3 | jack | 30 | 201 | 200 | hr | +| 3 | jack | 30 | 201 | 201 | it | +| 3 | jack | 30 | 201 | 202 | sale | +| 3 | jack | 30 | 201 | 203 | fd | +| 4 | alice | 24 | 202 | 200 | hr | +| 4 | alice | 24 | 202 | 201 | it | +| 4 | alice | 24 | 202 | 202 | sale | +| 4 | alice | 24 | 202 | 203 | fd | +| 5 | robin | 40 | 200 | 200 | hr | +| 5 | robin | 40 | 200 | 201 | it | +| 5 | robin | 40 | 200 | 202 | sale | +| 5 | robin | 40 | 200 | 203 | fd | +| 6 | wxin | 16 | 200 | 200 | hr | +| 6 | wxin | 16 | 200 | 201 | it | +| 6 | wxin | 16 | 200 | 202 | sale | +| 6 | wxin | 16 | 200 | 203 | fd | +| 7 | natasha | 28 | 204 | 200 | hr | +| 7 | natasha | 28 | 204 | 201 | it | +| 7 | natasha | 28 | 204 | 202 | sale | +| 7 | natasha | 28 | 204 | 203 | fd | ++--------+----------+------+---------+---------+-----------+ +28 rows in set (0.00 sec) +``` + +### 3. 复合条件连接查询 + +#### 案例一 + +​ 找出公司所有部门中年龄大于25岁的员工 + +​ 以内连接的方式查询employee6和department6表，并且employee6表中的age字段值必须大于25 + +```SQL +mysql> select emp_id,emp_name,dept_name from employee2,department2 where employee2.dept_id=department2.dept_id and age>25; ++--------+----------+-----------+ +| emp_id | emp_name | dept_name | ++--------+----------+-----------+ +| 5 | robin | hr | +| 2 | tom | it | +| 3 | jack | it | ++--------+----------+-----------+ +3 rows in set (0.00 sec) +``` + +#### 案例二 + +​ 以内连接的方式查询employee6和department6表，并且以age字段的升序方式显示 + +```SQL +mysql> select emp_id,emp_name,dept_name from employee2,department2 where employee2.dept_id=department2.dept_id order by age asc; ++--------+----------+-----------+ +| emp_id | emp_name | dept_name | ++--------+----------+-----------+ +| 6 | wxin | hr | +| 1 | | hr | +| 4 | alice | sale | +| 2 | tom | it | +| 3 | jack | it | +| 5 | robin | hr | ++--------+----------+-----------+ +6 rows in set (0.00 sec) +``` + +### 4. 子查询 + +​ 子查询是将一个查询语句嵌套在另一个查询语句中 + +​ 内层查询语句的查询结果，可以为外层查询语句提供查询条件 + +​ 子查询中可以包含：IN、NOT IN等关键字；还可以包含比较运算符：= 、 !=、> 、<等 + +#### 案例一 + +​ 带IN关键字的子查询；查询employee表，但dept_id必须在department表中出现过 + +```SQL +ysql> select * from employee2 where dept_id in (select dept_id from department2); ++--------+----------+------+---------+ +| emp_id | emp_name | age | dept_id | ++--------+----------+------+---------+ +| 1 | | 19 | 200 | +| 2 | tom | 26 | 201 | +| 3 | jack | 30 | 201 | +| 4 | alice | 24 | 202 | +| 5 | robin | 40 | 200 | +| 6 | wxin | 16 | 200 | ++--------+----------+------+---------+ +6 rows in set (0.00 sec) +``` + +#### 案例二 + +​ 带比较运算符的子查询；查询年龄大于等于25岁员工所在部门（查询老龄化的部门） + +```SQL +mysql> select dept_id,dept_name from department2 where dept_id in (select distinct dept_id from employee2 where age>=25); ++---------+-----------+ +| dept_id | dept_name | ++---------+-----------+ +| 201 | it | +| 200 | hr | ++---------+-----------+ +2 rows in set (0.00 sec) +``` \ No newline at end of file