买了网站 怎么做呢,全球速卖通官网,西宁网站建设公司排行,注册的网站清晰讲解SQL语句中的外连接#xff0c;通用于Mysql和Oracle#xff0c;全是干货哦直入主题#xff1a;我们做一个操作#xff0c;将员工SCOTT的部门去掉#xff0c;再次通过内连接查看数据#xff0c;看看会产生什么现象#xff1f;使用内连接#xff0c;查询数据问题通用于Mysql和Oracle全是干货哦直入主题我们做一个操作将员工SCOTT的部门去掉再次通过内连接查看数据看看会产生什么现象使用内连接查询数据问题找不到SCOTT员工了只有13条数据这显然不合理这就是内连接的缺点-- 内连接只显示匹配的数据-- 外连接显示匹配的数据还显示(部分或者全部)不匹配的数据那就是显示(全部)的数据(1)左外连接left join特点左表全部显示包括不匹配的数据右表进行匹配显示(2)右外连接right join右表全部显示包括不匹配的数据左表进行匹配显示(3)全外连接左表和右表的数据都会显示包括不匹配的数据注意在oracle中支持full join在mysql中不支持全外连接但是可以通过union 连接左外连接和右外连接达到全外连接的效果(4)自连接思路将一张表看做两张表 下级表 上级表举例显示没有上级的员工(king)知识扩展三表查询就是先查两张表将查出的结果看成一张表然后跟第三张表进行联合查询本质还是两表查询准备工作先添加一张薪资等级表第一步连接emp和dept表查询员工的编号、姓名、薪水、部门编号、部门名称第二步连接第三张表-- 查询员工的编号、姓名、薪水、部门编号、部门名称、薪水等级至此全部的表连接讲解完毕欢迎留言指出问题有则改之无则加勉下面分享所有的SQL语句复制代码create table DEPT(DEPTNO int(2) not null,DNAME VARCHAR(14),LOC VARCHAR(13));alter table DEPTadd constraint PK_DEPT primary key (DEPTNO);create table EMP(EMPNO int(4) primary key,ENAME VARCHAR(10),JOB VARCHAR(9),MGR int(4),HIREDATE DATE,SAL double(7,2),COMM double(7,2),DEPTNO int(2));alter table EMPadd constraint FK_DEPTNO foreign key (DEPTNO)references DEPT (DEPTNO);create table SALGRADE(GRADE int primary key,LOSAL double(7,2),HISAL double(7,2));create table BONUS(ENAME VARCHAR(10),JOB VARCHAR(9),SAL double(7,2),COMM double(7,2));commit;insert into DEPT (DEPTNO, DNAME, LOC)values (10, ACCOUNTING, NEW YORK);insert into DEPT (DEPTNO, DNAME, LOC)values (20, RESEARCH, DALLAS);insert into DEPT (DEPTNO, DNAME, LOC)values (30, SALES, CHICAGO);insert into DEPT (DEPTNO, DNAME, LOC)values (40, OPERATIONS, BOSTON);commit;insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7369, SMITH, CLERK, 7902, 1980-12-17, 800, null, 20);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7499, ALLEN, SALESMAN, 7698, 1981-02-20, 1600, 300, 30);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7521, WARD, SALESMAN, 7698, 1981-02-22, 1250, 500, 30);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7566, JONES, MANAGER, 7839, 1981-04-02, 2975, null, 20);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7654, MARTIN, SALESMAN, 7698, 1981-09-28, 1250, 1400, 30);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7698, BLAKE, MANAGER, 7839, 1981-05-01, 2850, null, 30);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7782, CLARK, MANAGER, 7839, 1981-06-09, 2450, null, 10);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7788, SCOTT, ANALYST, 7566, 1987-04-19, 3000, null, 20);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7839, KING, PRESIDENT, null, 1981-11-17, 5000, null, 10);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7844, TURNER, SALESMAN, 7698, 1981-09-08, 1500, 0, 30);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7876, ADAMS, CLERK, 7788, 1987-05-23, 1100, null, 20);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7900, JAMES, CLERK, 7698, 1981-12-03, 950, null, 30);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7902, FORD, ANALYST, 7566, 1981-12-03, 3000, null, 20);insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)values (7934, MILLER, CLERK, 7782, 1982-01-23, 1300, null, 10);commit;insert into SALGRADE (GRADE, LOSAL, HISAL)values (1, 700, 1200);insert into SALGRADE (GRADE, LOSAL, HISAL)values (2, 1201, 1400);insert into SALGRADE (GRADE, LOSAL, HISAL)values (3, 1401, 2000);insert into SALGRADE (GRADE, LOSAL, HISAL)values (4, 2001, 3000);insert into SALGRADE (GRADE, LOSAL, HISAL)values (5, 3001, 9999);commit;复制代码复制代码-- 一、内连接-- 交叉连接 cross JOIN-- 笛卡尔积现象SELECT *FROM empCROSS JOIN dept;-- 使用using只显示指定的同名字段SELECT *FROM emp eCROSS JOIN dept dUSING (deptno);-- 使用on指定不同名的字段SELECT *FROM emp eCROSS JOIN dept dON(e.deptno d.deptno);-- natural join 自然连接 所有同名列只显示一次SELECT *FROM empNATURAL JOIN dept;-- 二、外连接-- 左外连接 emp左表dept 右表左表全部显示包括没有数据的列SELECT *FROM emp e -- 左表LEFT JOIN dept d -- 右表on(e.deptno d.deptno);-- 右外连接 emp左表 dept右表右表全部显示包括没有数据的列SELECT *FROM emp e -- 左表RIGHT JOIN dept d -- 右表on(e.deptno d.deptno);-- 全外连接 oracle支持mysql不支持-- 可以通过关键字union进行解决,左表和右表的无对应数据列都会显示SELECT *FROM emp eLEFT JOIN dept don(e.deptno d.deptno);UNIONSELECT *FROM emp eRIGHT JOIN dept don(e.deptno d.deptno);-- 三、三表查询SELECT * FROM emp;SELECT * FROM dept;SELECT * FROM salgrade;-- 第一步先查两张表SELECT e.empno,e.ename,e.sal,d.deptno,d.dnameFROM emp eJOIN dept dON e.deptno d.deptno;-- 第二步连接第三张表SELECT e.empno, e.ename,e.sal,d.deptno,d.dname,sg.gradeFROM emp eJOIN dept dON e.deptno d.deptnoJOIN salgrade sgON e.sal BETWEEN sg.losal AND sg.hisal;-- 四、自连接-- 不是所有表都可以进行自连接只有表中的两列有关联关系才可以进行自连接查询-- 将自身分解成两张表SELECT e.empno,e.ename,e.mgr,m.empno,m.enameFROM emp eLEFT JOIN emp mON e.mgr m.empno;复制代码