一、多表查询
简言之，根据特定的连接条件从不同的表中获取所需的数据 笛卡尔集的产生条件： 省略连接条件 连接条件无效 第一个表中的所有行与第二个表中的所有行相连接
二、多表查询语法：
SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column1 = table2.column2; 但要注意where 不要省了，省略where 即为笛卡尔集，而且where 条件要有效， 两张表间有一个相同的字段，才好进行有效的多表查询 查询时列名前，加表名或表别名前辍（如果字段在两个表中是唯一的可以不加） 为了简化SQL书写，可为表名定义别名，格式：from 表名别名 如：from emp e,dept d 建议使用表的别名及表前缀，使用表别名可以简化查询，而使用表前缀则可以提高查询性能 例：查询每个员工的工号，姓名，工资，部门名和工作地点 select empno,ename,sal,dname,loc from emp,dept where emp.deptno=dept.deptno;
三、多表连接类型：
从数据显示方式来讲有：内连接和外连接。
内连接：只返回满足连接条件的数据。
外连接：除了返回满足连接条的行以外，还返回左（右）表中，不满足条件的行，称为左（右）连接
内连接*/
select empno,ename,sal,dname,loc from emp,dept
where emp.deptno=dept.deptno; --(Oracle 8i 及以前的写法)
--内连接的另一种写法：
select empno,ename,job,sal,dept.deptno,dname,loc
from emp join dept on (emp.deptno=dept.deptno); -- (SQL 99的写法)
外连接：
两个表的查询中，使用外连接可以查询另一个表或者两个中不满足连接条件的数据。
外连接的符号是（＋），（＋）要放在字段名后。（＋）对面的那个表，会全部显示。
外连接语法*/
SELECT table1.column, table2.column --右外连接
FROM table1, table2
WHERE table1.column(+) = table2.column;
SELECT table1.column, table2.column --左外连接
FROM table1, table2
WHERE table1.column = table2.column(+);
--例：
select empno,ename,job,sal,dept.deptno,dname,loc
from emp,dept
where emp.deptno(+)=dept.deptno; -- (Oracle 8i 及以前的写法)
--另一种写法（右连接）： -- (SQL 99的写法)
select empno,ename,job,sal,dept.deptno,dname,loc from emp right join dept on (emp.deptno=dept.deptno);
--左连接： (SQL 99的写法)
select empno,ename,job,sal,dept.deptno,dname,loc from emp left join dept on (emp.deptno=dept.deptno);
--全连接（满连接） (SQL 99的写法)
select empno,ename,job,sal,d.deptno,dname,loc from emp e full join dept d on (e.deptno=d.deptno);
自然连接 (SQL 99的写法)
以两个表具有相同的字段的所有列为基础，可采用自然连接（natural join)
它将选择两个表中那些在所有匹配的列中值相等的行。
如果列具有相同的名称，但数据类型能够不同，则会报错*/
select empno,ename,job,sal,deptno,dname,loc from emp natural join dept;
/*
自连接：
将自身表的一个镜像当另一个表来对待。*/
select ... from emp e,emp d ...
--下面列出SQL 99的语法供参考
SELECT table1.column, table2.column
FROM table1
[CROSS JOIN table2] |
[NATURAL JOIN table2] |
[JOIN table2 USING (column_name)] |
[JOIN table2
ON(table1.column_name = table2.column_name)] |
[LEFT|RIGHT|FULL OUTER JOIN table2
ON (table1.column_name = table2.column_name)];
--在下面的语法中
table1.column --指明从中检索数据的表和列
CROSS JOIN --返回两个表的笛卡尔集
NATURAL JOIN --根据相同的列名连接两个表
JOIN table
USING column_name --根据列名执行等值连接
JOIN table ON
table1.column_name --根据ON 子句中的条件执行等值连接
= table2.column_name
LEFT/RIGHT/FULL OUTER
/*
使用using子句创建连接
如果几个列具有相同的名称，但是数据类型不匹配，那么可以使用using子句来修改natural join
子句以指定要用于等值连接的列
在多个列匹配时，使用using 子句只匹配一个列
在引用列中不要使用表名或别名
natural join 和using 子句是互不相容的 */
--例：
SELECT l.city, d.department_name
FROM locations l JOIN departments d USING (location_id)
WHERE location_id = 1400;
--下面的语句无效，因为where 子句中限定了location_id
SELECT l.city, d.department_name
FROM locations l JOIN departments d USING (location_id)
WHERE d.location_id = 1400;
ORA-25154: column part of USING clause cannot have qualifier
--注意：
-- 两个表中名称相同的列在使用时不能有任何限定符，这一限制同样适用于natural join
四、演示： */
--笛卡尔集
SQL> select empno,ename,dname from emp,dept;
--使用cross join 实现交叉连接，即笛卡尔集
SQL> select empno,ename,dname from emp cross join dept;
EMPNO ENAME DNAME
---------- ---------- --------------
7369 SMITH ACCOUNTING
7499 ALLEN ACCOUNTING
7521 WARD ACCOUNTING
7566 JONES ACCOUNTING
7654 MARTIN ACCOUNTING
7698 BLAKE ACCOUNTING
7782 CLARK ACCOUNTING
7788 SCOTT ACCOUNTING
7839 KING ACCOUNTING
7844 TURNER ACCOUNTING
7876 ADAMS ACCOUNTING
--等值连接(Oracle 写法)
SQL> select empno,ename,dname from emp,dept where emp.deptno = dept.deptno;
EMPNO ENAME DNAME
---------- ---------- --------------
7369 SMITH RESEARCH
7499 ALLEN SALES
7876 ADAMS RESEARCH
--部分结果省略
7902 FORD RESEARCH
7934 MILLER ACCOUNTING
14 rows selected.
--等值连接(SQL 99 写法)
SQL> select e.empno,e.ename,d.dname from emp e
inner join dept d
on e.deptno = d.deptno;
EMPNO ENAME DNAME
---------- ---------- --------------
7369 SMITH RESEARCH
7499 ALLEN SALES
7876 ADAMS RESEARCH
--部分结果省略
7902 FORD RESEARCH
7934 MILLER ACCOUNTING
14 rows selected.
--注意：表别名不支持as 用法
SQL> select e.empno,e.ename,d.dname from emp as e
2 inner join dept d
3 on e.deptno = d.deptno;
select e.empno,e.ename,d.dname from emp as e
*
ERROR at line 1:
ORA-00933: SQL command not properly ended
--等值连接并增加条件
SQL> select e.empno,e.ename,d.dname from emp e,
2 dept d
3 where d.deptno = e.deptno
4 and e.ename = ‘SCOTT‘;
EMPNO ENAME DNAME
---------- ---------- --------------
7788 SCOTT RESEARCH
--非等值连接
--查询雇员的姓名、薪水、级别且部门为的记录
SQL> select ename,sal,grade
2 from emp,salgrade
3 where sal between losal and hisal
4 and emp.deptno = 20;
ENAME SAL GRADE
---------- ---------- ----------
SCOTT 3000 4
FORD 3000 4
JONES 2975 4
ADAMS 1100 1
SMITH 800 1
--使用SQL 99写法实现上述功能
SQL> select e.ename,e.sal,s.grade
2 from emp e
3 join salgrade s
4 on e.sal between losal and hisal
5 and e.deptno = 20;
ENAME SAL GRADE
---------- ---------- ----------
SCOTT 3000 4
FORD 3000 4
JONES 2975 4
ADAMS 1100 1
SMITH 800 1
--右外连接
--注意：右外连接时，加号在等号的左边
--可以看到，左表emp中的列有为空值的
SQL> select e.ename,e.deptno,d.dname from emp e,dept d where e.deptno(+) = d.deptno;
idle> select e.ename,e.deptno,d.dname from emp e,dept d where e.deptno(+) = d.deptno; ENAME DEPTNO DNAME ---------- ---------- -------------- CLARK 10 ACCOUNTING KING 10 ACCOUNTING MILLER 10 ACCOUNTING JONES 20 RESEARCH FORD 20 RESEARCH ADAMS 20 RESEARCH SMITH 20 RESEARCH SCOTT 20 RESEARCH WARD 30 SALES TURNER 30 SALES ALLEN 30 SALES JAMES 30 SALES BLAKE 30 SALES MARTIN 30 SALES OPERATIONS 15 rows selected.
--使用SQL 99写法实现右外连接
SQL> select e.ename,e.deptno,d.dname from emp e right join dept d on e.deptno = d.deptno ;
idle> select e.ename,e.deptno,d.dname from emp e right join dept d on e.deptno = d.deptno ; ENAME DEPTNO DNAME ---------- ---------- -------------- CLARK 10 ACCOUNTING KING 10 ACCOUNTING MILLER 10 ACCOUNTING JONES 20 RESEARCH FORD 20 RESEARCH ADAMS 20 RESEARCH SMITH 20 RESEARCH SCOTT 20 RESEARCH WARD 30 SALES TURNER 30 SALES ALLEN 30 SALES JAMES 30 SALES BLAKE 30 SALES MARTIN 30 SALES OPERATIONS 15 rows selected.
--左外连接
--注意：左外连接时，加号在等号的右边
select d.dname,e.ename,e.deptno from dept d,emp e where d.deptno = e.deptno(+) order by d.deptno;
idle> select d.dname,e.ename,e.deptno from dept d,emp e where d.deptno = e.deptno(+) order by d.deptno; DNAME ENAME DEPTNO -------------- ---------- ---------- ACCOUNTING CLARK 10 ACCOUNTING KING 10 ACCOUNTING MILLER 10 RESEARCH JONES 20 RESEARCH FORD 20 RESEARCH ADAMS 20 RESEARCH SMITH 20 RESEARCH SCOTT 20 SALES WARD 30 SALES TURNER 30 SALES ALLEN 30 SALES JAMES 30 SALES BLAKE 30 SALES MARTIN 30 OPERATIONS 15 rows selected.
--使用SQL 99写法实现左外连接
SQL> select d.dname,e.ename,e.deptno from dept d left join emp e on d.deptno = e.deptno order by d.deptno;
idle> select d.dname,e.ename,e.deptno from dept d left join emp e on d.deptno = e.deptno order by d.deptno; DNAME ENAME DEPTNO -------------- ---------- ---------- ACCOUNTING CLARK 10 ACCOUNTING KING 10 ACCOUNTING MILLER 10 RESEARCH JONES 20 RESEARCH FORD 20 RESEARCH ADAMS 20 RESEARCH SMITH 20 RESEARCH SCOTT 20 SALES WARD 30 SALES TURNER 30 SALES ALLEN 30 SALES JAMES 30 SALES BLAKE 30 SALES MARTIN 30 OPERATIONS 15 rows selected.
--自连接
SQL> select e.ename || ‘ works for ‘ || m.ename from emp e,emp m where e.empno = m.mgr;
idle> select e.ename || ‘ works for ‘ || m.ename from emp e,emp m where e.empno = m.mgr; E.ENAME||‘WORKSFOR‘||M.ENAME ------------------------------- JONES works for FORD JONES works for SCOTT BLAKE works for TURNER BLAKE works for ALLEN BLAKE works for WARD BLAKE works for JAMES BLAKE works for MARTIN CLARK works for MILLER SCOTT works for ADAMS KING works for BLAKE KING works for JONES KING works for CLARK FORD works for SMITH 13 rows selected.
--自然连接
SQL> select empno,ename,job,deptno,dname,loc from emp natural join dept;
idle> select empno,ename,job,deptno,dname,loc from emp natural join dept;
EMPNO ENAME JOB DEPTNO DNAME LOC
---------- ---------- --------- ---------- -------------- -------------
7782 CLARK MANAGER 10 ACCOUNTING NEW YORK
7839 KING PRESIDENT 10 ACCOUNTING NEW YORK
7934 MILLER CLERK 10 ACCOUNTING NEW YORK
7566 JONES MANAGER 20 RESEARCH DALLAS
7902 FORD ANALYST 20 RESEARCH DALLAS
7876 ADAMS CLERK 20 RESEARCH DALLAS
7369 SMITH CLERK 20 RESEARCH DALLAS
7788 SCOTT ANALYST 20 RESEARCH DALLAS
7521 WARD SALESMAN 30 SALES CHICAGO
7844 TURNER SALESMAN 30 SALES CHICAGO
7499 ALLEN SALESMAN 30 SALES CHICAGO
7900 JAMES CLERK 30 SALES CHICAGO
7698 BLAKE MANAGER 30 SALES CHICAGO
7654 MARTIN SALESMAN 30 SALES CHICAGO
14 rows selected.
--使用using 子句创建连接
select e.empno,e.ename,d.dname,d.loc
from emp e
join dept d
using (deptno)
where deptno in (20,40);
idle> select e.empno,e.ename,d.dname,d.loc
from emp e
join dept d
using (deptno)
where deptno in (20,40); 2 3 4 5
EMPNO ENAME DNAME LOC
---------- ---------- -------------- -------------
7369 SMITH RESEARCH DALLAS
7566 JONES RESEARCH DALLAS
7788 SCOTT RESEARCH DALLAS
7876 ADAMS RESEARCH DALLAS
7902 FORD RESEARCH DALLAS
