java基础-多表查询

多表查询：

    * 查询语法：

        select

            列名列表

        from

            表名列表

        where.... * 准备sql

        # 创建部门表

        CREATE TABLE dept(

            id INT PRIMARY KEY AUTO_INCREMENT,

            NAME VARCHAR(20)

        );

        INSERT INTO dept (NAME) VALUES ('开发部'),('市场部'),('财务部');

        # 创建员工表

        CREATE TABLE emp (

            id INT PRIMARY KEY AUTO_INCREMENT,

            NAME VARCHAR(10),

            gender CHAR(1), -- 性别

            salary DOUBLE, -- 工资

            join_date DATE, -- 入职日期

            dept_id INT,

            FOREIGN KEY (dept_id) REFERENCES dept(id) -- 外键，关联部门表(部门表的主键)

        );

        INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES('孙悟空','男',7200,'2013-02-24',1);

        INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES('猪八戒','男',3600,'2010-12-02',2);

        INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES('唐僧','男',9000,'2008-08-08',2);

        INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES('白骨精','女',5000,'2015-10-07',3);

        INSERT INTO emp(NAME,gender,salary,join_date,dept_id) VALUES('蜘蛛精','女',4500,'2011-03-14',1); * 笛卡尔积：

        * 有两个集合A,B .取这两个集合的所有组成情况。

        * 要完成多表查询，需要消除无用的数据

    * 多表查询的分类：

        1. 内连接查询：

            1. 隐式内连接：使用where条件消除无用数据

                * 例子：

                -- 查询所有员工信息和对应的部门信息

                SELECT * FROM emp,dept WHERE emp.`dept_id` = dept.`id`;

                -- 查询员工表的名称，性别。部门表的名称

                SELECT emp.name,emp.gender,dept.name FROM emp,dept WHERE emp.`dept_id` = dept.`id`;

                SELECT

                    t1.name, -- 员工表的姓名

                    t1.gender,-- 员工表的性别

                    t2.name -- 部门表的名称

                FROM

                    emp t1,

                    dept t2

                WHERE

                    t1.`dept_id` = t2.`id`;

            2. 显式内连接：

                * 语法： select 字段列表 from 表名1 [inner] join 表名2 on 条件

                * 例如：

                    * SELECT * FROM emp INNER JOIN dept ON emp.`dept_id` = dept.`id`;    

                    * SELECT * FROM emp JOIN dept ON emp.`dept_id` = dept.`id`;    

            3. 内连接查询：

                1. 从哪些表中查询数据

                2. 条件是什么

                3. 查询哪些字段

        2. 外链接查询：

            1. 左外连接：

                * 语法：select 字段列表 from 表1 left [outer] join 表2 on 条件；

                * 查询的是左表所有数据以及其交集部分。

                * 例子：

                    -- 查询所有员工信息，如果员工有部门，则查询部门名称，没有部门，则不显示部门名称

                    SELECT     t1.*,t2.`name` FROM emp t1 LEFT JOIN dept t2 ON t1.`dept_id` = t2.`id`;

            2. 右外连接：

                * 语法：select 字段列表 from 表1 right [outer] join 表2 on 条件；

                * 查询的是右表所有数据以及其交集部分。

                * 例子：

                    SELECT     * FROM dept t2 RIGHT JOIN emp t1 ON t1.`dept_id` = t2.`id`;

        3. 子查询：

            * 概念：查询中嵌套查询，称嵌套查询为子查询。

                -- 查询工资最高的员工信息

                -- 1 查询最高的工资是多少 9000

                SELECT MAX(salary) FROM emp;

                -- 2 查询员工信息，并且工资等于9000的

                SELECT * FROM emp WHERE emp.`salary` = 9000;

                -- 一条sql就完成这个操作。子查询

                SELECT * FROM emp WHERE emp.`salary` = (SELECT MAX(salary) FROM emp);

            * 子查询不同情况

                1. 子查询的结果是单行单列的：

                    * 子查询可以作为条件，使用运算符去判断。 运算符： > >= < <= =

                    *

                    -- 查询员工工资小于平均工资的人

                    SELECT * FROM emp WHERE emp.salary < (SELECT AVG(salary) FROM emp);

                2. 子查询的结果是多行单列的：

                    * 子查询可以作为条件，使用运算符in来判断

                    -- 查询'财务部'和'市场部'所有的员工信息

                    SELECT id FROM dept WHERE NAME = '财务部' OR NAME = '市场部';

                    SELECT * FROM emp WHERE dept_id = 3 OR dept_id = 2;

                    -- 子查询

                    SELECT * FROM emp WHERE dept_id IN (SELECT id FROM dept WHERE NAME = '财务部' OR NAME = '市场部');

                3. 子查询的结果是多行多列的：

                    * 子查询可以作为一张虚拟表参与查询

                    -- 查询员工入职日期是2011-11-11日之后的员工信息和部门信息

                    -- 子查询

                    SELECT * FROM dept t1 ,(SELECT * FROM emp WHERE emp.`join_date` > '2011-11-11') t2

                    WHERE t1.id = t2.dept_id;

                    -- 普通内连接

                    SELECT * FROM emp t1,dept t2 WHERE t1.`dept_id` = t2.`id` AND t1.`join_date` >  '2011-11-11'

        * 多表查询练习

                -- 部门表

                CREATE TABLE dept (

                  id INT PRIMARY KEY PRIMARY KEY, -- 部门id

                  dname VARCHAR(50), -- 部门名称

                  loc VARCHAR(50) -- 部门所在地

                );

                -- 添加4个部门

                INSERT INTO dept(id,dname,loc) VALUES

                (10,'教研部','北京'),

                (20,'学工部','上海'),

                (30,'销售部','广州'),

                (40,'财务部','深圳');

                -- 职务表，职务名称，职务描述

                CREATE TABLE job (

                  id INT PRIMARY KEY,

                  jname VARCHAR(20),

                  description VARCHAR(50)

                );

                -- 添加4个职务

                INSERT INTO job (id, jname, description) VALUES

                (1, '董事长', '管理整个公司，接单'),

                (2, '经理', '管理部门员工'),

                (3, '销售员', '向客人推销产品'),

                (4, '文员', '使用办公软件');

                -- 员工表

                CREATE TABLE emp (

                  id INT PRIMARY KEY, -- 员工id

                  ename VARCHAR(50), -- 员工姓名

                  job_id INT, -- 职务id

                  mgr INT , -- 上级领导

                  joindate DATE, -- 入职日期

                  salary DECIMAL(7,2), -- 工资

                  bonus DECIMAL(7,2), -- 奖金

                  dept_id INT, -- 所在部门编号

                  CONSTRAINT emp_jobid_ref_job_id_fk FOREIGN KEY (job_id) REFERENCES job (id),

                  CONSTRAINT emp_deptid_ref_dept_id_fk FOREIGN KEY (dept_id) REFERENCES dept (id)

                );

                -- 添加员工

                INSERT INTO emp(id,ename,job_id,mgr,joindate,salary,bonus,dept_id) VALUES

                (1001,'孙悟空',4,1004,'2000-12-17','8000.00',NULL,20),

                (1002,'卢俊义',3,1006,'2001-02-20','16000.00','3000.00',30),

                (1003,'林冲',3,1006,'2001-02-22','12500.00','5000.00',30),

                (1004,'唐僧',2,1009,'2001-04-02','29750.00',NULL,20),

                (1005,'李逵',4,1006,'2001-09-28','12500.00','14000.00',30),

                (1006,'宋江',2,1009,'2001-05-01','28500.00',NULL,30),

                (1007,'刘备',2,1009,'2001-09-01','24500.00',NULL,10),

                (1008,'猪八戒',4,1004,'2007-04-19','30000.00',NULL,20),

                (1009,'罗贯中',1,NULL,'2001-11-17','50000.00',NULL,10),

                (1010,'吴用',3,1006,'2001-09-08','15000.00','0.00',30),

                (1011,'沙僧',4,1004,'2007-05-23','11000.00',NULL,20),

                (1012,'李逵',4,1006,'2001-12-03','9500.00',NULL,30),

                (1013,'小白龙',4,1004,'2001-12-03','30000.00',NULL,20),

                (1014,'关羽',4,1007,'2002-01-23','13000.00',NULL,10);

                -- 工资等级表

                CREATE TABLE salarygrade (

                  grade INT PRIMARY KEY,   -- 级别

                  losalary INT,  -- 最低工资

                  hisalary INT -- 最高工资

                );

                -- 添加5个工资等级

                INSERT INTO salarygrade(grade,losalary,hisalary) VALUES

                (1,7000,12000),

                (2,12010,14000),

                (3,14010,20000),

                (4,20010,30000),

                (5,30010,99990);

                -- 需求：

                -- 1.查询所有员工信息。查询员工编号，员工姓名，工资，职务名称，职务描述

                /*

                    分析：

                        1.员工编号，员工姓名，工资，需要查询emp表  职务名称，职务描述 需要查询job表

                        2.查询条件 emp.job_id = job.id

                */

                SELECT

                    t1.`id`, -- 员工编号

                    t1.`ename`, -- 员工姓名

                    t1.`salary`,-- 工资

                    t2.`jname`, -- 职务名称

                    t2.`description` -- 职务描述

                FROM

                    emp t1, job t2

                WHERE

                    t1.`job_id` = t2.`id`;

                -- 2.查询员工编号，员工姓名，工资，职务名称，职务描述，部门名称，部门位置

                /*

                    分析：

                        1. 员工编号，员工姓名，工资 emp  职务名称，职务描述 job  部门名称，部门位置 dept

                        2. 条件： emp.job_id = job.id and emp.dept_id = dept.id

                */

                SELECT

                    t1.`id`, -- 员工编号

                    t1.`ename`, -- 员工姓名

                    t1.`salary`,-- 工资

                    t2.`jname`, -- 职务名称

                    t2.`description`, -- 职务描述

                    t3.`dname`, -- 部门名称

                    t3.`loc` -- 部门位置

                FROM

                    emp t1, job t2,dept t3

                WHERE

                    t1.`job_id` = t2.`id` AND t1.`dept_id` = t3.`id`;

                -- 3.查询员工姓名，工资，工资等级

                /*

                    分析：

                        1.员工姓名，工资 emp  工资等级 salarygrade

                        2.条件 emp.salary >= salarygrade.losalary and emp.salary <= salarygrade.hisalary

                            emp.salary BETWEEN salarygrade.losalary and salarygrade.hisalary

                */

                SELECT

                    t1.ename ,

                    t1.`salary`,

                    t2.*

                FROM emp t1, salarygrade t2

                WHERE t1.`salary` BETWEEN t2.`losalary` AND t2.`hisalary`;

                -- 4.查询员工姓名，工资，职务名称，职务描述，部门名称，部门位置，工资等级

                /*

                    分析：

                        1. 员工姓名，工资 emp ， 职务名称，职务描述 job 部门名称，部门位置，dept  工资等级 salarygrade

                        2. 条件： emp.job_id = job.id and emp.dept_id = dept.id and emp.salary BETWEEN salarygrade.losalary and salarygrade.hisalary

                */

                SELECT

                    t1.`ename`,

                    t1.`salary`,

                    t2.`jname`,

                    t2.`description`,

                    t3.`dname`,

                    t3.`loc`,

                    t4.`grade`

                FROM

                    emp t1,job t2,dept t3,salarygrade t4

                WHERE

                    t1.`job_id` = t2.`id`

                    AND t1.`dept_id` = t3.`id`

                    AND t1.`salary` BETWEEN t4.`losalary` AND t4.`hisalary`;

                -- 5.查询出部门编号、部门名称、部门位置、部门人数

                /*

                    分析：

                        1.部门编号、部门名称、部门位置 dept 表。 部门人数 emp表

                        2.使用分组查询。按照emp.dept_id完成分组，查询count(id)

                        3.使用子查询将第2步的查询结果和dept表进行关联查询

                */

                SELECT

                    t1.`id`,t1.`dname`,t1.`loc` , t2.total

                FROM

                    dept t1,

                    (SELECT

                        dept_id,COUNT(id) total

                    FROM

                        emp

                    GROUP BY dept_id) t2

                WHERE t1.`id` = t2.dept_id;

                -- 6.查询所有员工的姓名及其直接上级的姓名,没有领导的员工也需要查询

                /*

                    分析：

                        1.姓名 emp， 直接上级的姓名 emp

                            * emp表的id 和 mgr 是自关联

                        2.条件 emp.id = emp.mgr

                        3.查询左表的所有数据，和 交集数据

                            * 使用左外连接查询

                */

                /*

                select

                    t1.ename,

                    t1.mgr,

                    t2.`id`,

                    t2.ename

                from emp t1, emp t2

                where t1.mgr = t2.`id`;

                */

                SELECT

                    t1.ename,

                    t1.mgr,

                    t2.`id`,

                    t2.`ename`

                FROM emp t1

                LEFT JOIN emp t2

                ON t1.`mgr` = t2.`id`;

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