操做的表
mysql> select * from emp;mysql
+-------+--------+-----------+------+------------+---------+---------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+--------+-----------+------+------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | +-------+--------+-----------+------+------------+---------+---------+--------+
其每一个字段的字段名分别为员工编号、员工姓名、员工职位、员工上级领导编号、员工雇佣日期,员工薪水、员工津贴、员工部门编号。web
mysql> select * from dept;sql
+--------+------------+----------+ | DEPTNO | DNAME | LOC | +--------+------------+----------+ | 10 | ACCOUNTING | NEW YORK | | 20 | RESEARCH | DALLAS | | 30 | SALES | CHICAGO | | 40 | OPERATIONS | BOSTON | +--------+------------+----------+
其每一个字段的字段名分别部门编号、部门名称、位置。
mysql> select * from salgrade;shell
+-------+-------+-------+ | GRADE | LOSAL | HISAL | +-------+-------+-------+ | 1 | 700 | 1200 | | 2 | 1201 | 1400 | | 3 | 1401 | 2000 | | 4 | 2001 | 3000 | | 5 | 3001 | 9999 | +-------+-------+-------+
其中每一个字段的字段名分别为等级、该等级薪水下限、该等级薪水上限数据库
在实际开发中,大部分的状况下都不是从单表中查询数据,通常都是多张表联合查询取出最终的结果。
在实际开发中,通常一个业务都会对应多张表,好比:学生和班级,起码两张表。svg
stuno | name | classno | classname |
---|---|---|---|
1 | zs | 1 | 北京大兴区亦庄经济技术开发区第二中学高三1班 |
2 | ls | 1 | 北京大兴区亦庄经济技术开发区第二中学高三1班 |
… | … | … | … |
学生和班级信息存储到一张表中,结果就像上面同样,数据会存在大量的重复,致使数据的冗余。spa
根据语法出现的年代来划分的话,包括:
SQL92(一些老的DBA可能还在使用这种语法。DBA:DataBase Administrator,数据库管理员)
SQL99(比较新的语法)
根据表的链接方式来划分,包括:
内链接:
等值链接(靠等值关系)
非等值链接
自链接
外链接:
左外链接(左链接)
右外链接(右链接)
全链接(这个不讲,不多用!)code
在表的链接查询方面有一种现象被称为:笛卡尔积现象。(笛卡尔乘积现象)
案例:找出每个员工的部门名称,要求显示员工名和部门名。
员工表empxml
+-------+--------+-----------+------+------------+---------+---------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+--------+-----------+------+------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | +-------+--------+-----------+------+------------+---------+---------+--------+
部门表depttoken
+--------+------------+----------+ | DEPTNO | DNAME | LOC | +--------+------------+----------+ | 10 | ACCOUNTING | NEW YORK | | 20 | RESEARCH | DALLAS | | 30 | SALES | CHICAGO | | 40 | OPERATIONS | BOSTON | +--------+------------+----------+
select ename,dname from emp,dept; //产生56条记录
+--------+------------+ | ename | dname | +--------+------------+ | SMITH | ACCOUNTING | | SMITH | RESEARCH | | SMITH | SALES | | SMITH | OPERATIONS | | ALLEN | ACCOUNTING | | ALLEN | RESEARCH | | ALLEN | SALES | | ALLEN | OPERATIONS | | WARD | ACCOUNTING | | WARD | RESEARCH | | WARD | SALES | | WARD | OPERATIONS | ........
笛卡尔积现象:当两张表进行链接查询的时候,没有任何条件进行限制,最终的查询结果条数是两张表记录条数的乘积。
A表和B表链接得时候,从A表中每拿出一条记录,B表中得全部记录都与该记录匹配。
关于表的别名:
select e.ename,d.dname from emp e,dept d;
表的别名有什么好处?
第一:执行效率高。
第二:可读性好。
由于两张表的属性名可能会同样
固然是加条件进行过滤。
思考:避免了笛卡尔积现象,会减小记录的匹配次数吗?
不会,次数仍是56次。只不过显示的是有效记录。
案例:找出每个员工的部门名称,要求显示员工名和部门名。
select e.ename,d.dname from emp e , dept d where e.deptno = d.deptno; //SQL92,之后不用。
+--------+------------+ | ename | dname | +--------+------------+ | SMITH | RESEARCH | | ALLEN | SALES | | WARD | SALES | | JONES | RESEARCH | | MARTIN | SALES | | BLAKE | SALES | | CLARK | ACCOUNTING | | SCOTT | RESEARCH | | KING | ACCOUNTING | | TURNER | SALES | | ADAMS | RESEARCH | | JAMES | SALES | | FORD | RESEARCH | | MILLER | ACCOUNTING | +--------+------------+
案例:查询每一个员工的部门名称,要求显示员工名和部门名。
SQL92:(太老,不用了)
select e.ename,d.dname from emp e, dept d where e.deptno = d.deptno;
select e.ename,d.dname from emp e join dept d ON e.DEPTNO=d.DEPTNO;
// inner能够省略的,带着inner目的是可读性好一些。 select e.ename,d.dname from emp e inner join dept d on e.deptno = d.deptno; 语法: ... A join B on 链接条件 where ...
SQL99语法结构更清晰一些:表的链接条件和后来的where条件分离了。
表链接和数据过滤混合了
内链接之非等值链接:最大的特色是:链接条件中的关系是非等量关系。
案例:找出每一个员工的工资等级,要求显示员工名、工资、工资等级。
查询员工名和工资的信息
select ename,sal from emp;
+--------+---------+ | ename | sal | +--------+---------+ | SMITH | 800.00 | | ALLEN | 1600.00 | | WARD | 1250.00 | | JONES | 2975.00 | | MARTIN | 1250.00 | | BLAKE | 2850.00 | | CLARK | 2450.00 | | SCOTT | 3000.00 | | KING | 5000.00 | | TURNER | 1500.00 | | ADAMS | 1100.00 | | JAMES | 950.00 | | FORD | 3000.00 | | MILLER | 1300.00 | +--------+---------+
工资等级信息表salgrade;
+-------+-------+-------+ | GRADE | LOSAL | HISAL | +-------+-------+-------+ | 1 | 700 | 1200 | | 2 | 1201 | 1400 | | 3 | 1401 | 2000 | | 4 | 2001 | 3000 | | 5 | 3001 | 9999 | +-------+-------+-------+
按照题设条件,这里Smith先生得工资应该与Grade1进行匹配,他得工资是800位于losal和hisal之间
select e.ename,e.sal,s.grade from emp e join salgrade s on e.sal BETWEEN s.LOSAL AND s.HISAL
+--------+---------+-------+ | ename | sal | grade | +--------+---------+-------+ | SMITH | 800.00 | 1 | | ALLEN | 1600.00 | 3 | | WARD | 1250.00 | 2 | | JONES | 2975.00 | 4 | | MARTIN | 1250.00 | 2 | | BLAKE | 2850.00 | 4 | | CLARK | 2450.00 | 4 | | SCOTT | 3000.00 | 4 | | KING | 5000.00 | 5 | | TURNER | 1500.00 | 3 | | ADAMS | 1100.00 | 1 | | JAMES | 950.00 | 1 | | FORD | 3000.00 | 4 | | MILLER | 1300.00 | 2 | +--------+---------+-------+
自链接:最大的特色是:一张表看作两张表。本身链接本身。
案例:找出每一个员工的上级领导,要求显示员工名和对应的领导名。
mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+--------+-----------+------+------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | +-------+--------+-----------+------+------------+---------+---------+--------+
其中,MGR表示上级领导的编号
如smith的mgr是7902,他的领导就是FORD.
员工的领导编号 = 领导的员工编号
select a.ename as '员工名',b.ename as '领导名' from emp a inner join emp b on a.mgr = b.empno;
+--------+--------+ | 员工名 | 领导名 | +--------+--------+ | SMITH | FORD | | ALLEN | BLAKE | | WARD | BLAKE | | JONES | KING | | MARTIN | BLAKE | | BLAKE | KING | | CLARK | KING | | SCOTT | JONES | | TURNER | BLAKE | | ADAMS | SCOTT | | JAMES | BLAKE | | FORD | JONES | | MILLER | CLARK | +--------+--------+
找出等量关系是关键
注意:此时只有13条记录,缺乏一条King的记录,他的领导为空
什么是外链接,和内链接有什么区别?
内链接:
假设A和B表进行链接,使用内链接的话,凡是A表和B表可以匹配上的记录查询出来,这就是内链接。
AB两张表没有主副之分,两张表是平等的。
外链接:
假设A和B表进行链接,使用外链接的话,AB两张表中有一张表是主表,一张表是副表,主要查询主表中的数据,捎带着查询副表,当副表中的数据没有和主表中的数据匹配上,副表自动模拟出NULL与之匹配。
外链接的分类?
左外链接(左链接):表示左边的这张表是主表。
右外链接(右链接):表示右边的这张表是主表。
左链接有右链接的写法,右链接也会有对应的左链接的写法。
案例:找出每一个员工的上级领导?(全部员工必须所有查询出来。)
外链接主表全部数据都要查出来。使用内链接的时候King(领导为NULL)
left join左边是主表,主表数据不管如何都要查出来,就算匹配不上,也要查出来。
select a.ename as '员工名',b.ename as '领导名' from emp a left join emp b on a.mgr = b.empno;
+--------+--------+ | 员工名 | 领导名 | +--------+--------+ | SMITH | FORD | | ALLEN | BLAKE | | WARD | BLAKE | | JONES | KING | | MARTIN | BLAKE | | BLAKE | KING | | CLARK | KING | | SCOTT | JONES | | KING | NULL | | TURNER | BLAKE | | ADAMS | SCOTT | | JAMES | BLAKE | | FORD | JONES | | MILLER | CLARK | +--------+--------+
副表匹配不上。
// outer是能够省略的。 select a.ename '员工', b.ename '领导' from emp a left outer join emp b on a.mgr = b.empno; 外链接:(右外链接/右链接) select a.ename '员工', b.ename '领导' from emp b right join emp a on a.mgr = b.empno;
inner和outer均可以省略,区份内外链接主要是看有没有right和left
带inner和outer可读性好
开发中使用外链接更多。
匹配不上,不能让数据丢失。
外链接最重要的特色是主表中的数据所有查询出来
案例:找出哪一个部门没有员工
select * from emp;
mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+--------+-----------+------+------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | +-------+--------+-----------+------+------------+---------+---------+--------+
select * from dept;
+--------+------------+----------+ | DEPTNO | DNAME | LOC | +--------+------------+----------+ | 10 | ACCOUNTING | NEW YORK | | 20 | RESEARCH | DALLAS | | 30 | SALES | CHICAGO | | 40 | OPERATIONS | BOSTON | +--------+------------+----------+
SELECT d.* FROM dept d left JOIN emp e on d.DEPTNO=e.DEPTNO WHERE e.empno is null;
+--------+------------+--------+ | DEPTNO | DNAME | LOC | +--------+------------+--------+ | 40 | OPERATIONS | BOSTON | +--------+------------+--------+
案例:找出每个员工的部门名称以及工资等级。
注意,解释一下:
.... A join B join C on ...
表示:A表和B表先进行表链接,链接以后A表继续和C表进行链接。
或者A和B之间链接的结果再和C链接
select e.ename,d.dname,s.grade from emp e join dept d on e.deptno = d.deptno join salgrade s on e.sal between s.losal and s.hisal;
+--------+------------+-------+ | ename | dname | grade | +--------+------------+-------+ | SMITH | RESEARCH | 1 | | ADAMS | RESEARCH | 1 | | JAMES | SALES | 1 | | MILLER | ACCOUNTING | 2 | | WARD | SALES | 2 | | MARTIN | SALES | 2 | | ALLEN | SALES | 3 | | TURNER | SALES | 3 | | CLARK | ACCOUNTING | 4 | | JONES | RESEARCH | 4 | | SCOTT | RESEARCH | 4 | | FORD | RESEARCH | 4 | | BLAKE | SALES | 4 | | KING | ACCOUNTING | 5 | +--------+------------+-------+
案例:找出每个员工的部门名称、工资等级、以及上级领导。
这里意味着没有领导的king也必须查出来。
select e.ename '员工',d.dname,s.grade,e1.ename '领导' from emp e join dept d on e.deptno = d.deptno join salgrade s on e.sal between s.losal and s.hisal left join emp e1 on e.mgr = e1.empno;
+--------+------------+-------+-------+ | 员工 | dname | grade | 领导 | +--------+------------+-------+-------+ | SMITH | RESEARCH | 1 | FORD | | ADAMS | RESEARCH | 1 | SCOTT | | JAMES | SALES | 1 | BLAKE | | WARD | SALES | 2 | BLAKE | | MARTIN | SALES | 2 | BLAKE | | MILLER | ACCOUNTING | 2 | CLARK | | ALLEN | SALES | 3 | BLAKE | | TURNER | SALES | 3 | BLAKE | | JONES | RESEARCH | 4 | KING | | BLAKE | SALES | 4 | KING | | CLARK | ACCOUNTING | 4 | KING | | SCOTT | RESEARCH | 4 | JONES | | FORD | RESEARCH | 4 | JONES | | KING | ACCOUNTING | 5 | NULL | +--------+------------+-------+-------+
注意,最后外链接的位置。