表连接分为：CROSS JOIN、INNERT JOIN、OUTER JOIN、SELF JOIN。
一、CROSS JOIN 交叉连接
CROSS JOIN连接用于生成两张表的笛卡尔集。
1、返回的记录数为两个表的记录数乘积。比如，A表有n条记录，B表有m条记录，则返回n*m条记录。
2、将A表的所有行分别与B表的所有行进行连接。
准备测试数据：
SQL> conn scott/tiger
Connected.
SQL> create table emp2 as select * from emp;
Table created.
SQL> create table dept2 as select * from dept;
Table created.
SQL> insert into dept2 values(60,'HR','A City');
1 row created.
SQL> insert into emp2(empno,ename,sal) values(1234,'test',4000);
1 row created.
SQL> commit;
Commit complete.
SQL> select * from emp2;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- -------------------- ------------------ ---------- ------------ ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
1234 test 4000
15 rows selected.
SQL> select * from dept2;
DEPTNO DNAME LOC
---------- ---------------------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
60 HR A City
5 rows selected.
-- cross join,返回 15 * 5行记录。
SQL> SELECT ename,dname
2 FROM emp2 CROSS JOIN dept2;
ENAME DNAME
-------------------- ----------------------------
SMITH ACCOUNTING
ALLEN ACCOUNTING
WARD ACCOUNTING
JONES ACCOUNTING
MARTIN ACCOUNTING
BLAKE ACCOUNTING
...
CLARK HR
SCOTT HR
KING HR
TURNER HR
ADAMS HR
JAMES HR
FORD HR
MILLER HR
test HR
75 rows selected.
二、INNER JOIN 内连接
内连接就是关联的两张或多张表中，根据关联条件，显示所有匹配的记录，匹配不上的，不显示。
以下两种是标准写法：
SQL> SELECT ename,sal
2 FROM emp2 e INNER JOIN dept2 d
3 ON e.deptno=d.deptno;
ENAME SAL
-------------------- ----------
SMITH 800
ALLEN 1600
WARD 1250
JONES 2975
MARTIN 1250
BLAKE 2850
CLARK 2450
SCOTT 3000
KING 5000
TURNER 1500
ADAMS 1100
JAMES 950
FORD 3000
MILLER 1300
14 rows selected.
--也可以省略inner关键字，直接写join
SQL> SELECT ename,sal
2 FROM emp2 e JOIN dept2 d
3 ON e.deptno=d.deptno;
ENAME SAL
-------------------- ----------
SMITH 800
ALLEN 1600
WARD 1250
JONES 2975
MARTIN 1250
BLAKE 2850
CLARK 2450
SCOTT 3000
KING 5000
TURNER 1500
ADAMS 1100
JAMES 950
FORD 3000
MILLER 1300
14 rows selected.
--如果两张表的关联字段名相同，也可以使用USING子句。
SQL> SELECT ename,sal
2 FROM emp2 e JOIN dept2 d
3 USING(deptno);
ENAME SAL
-------------------- ----------
SMITH 800
ALLEN 1600
WARD 1250
JONES 2975
MARTIN 1250
BLAKE 2850
CLARK 2450
SCOTT 3000
KING 5000
TURNER 1500
ADAMS 1100
JAMES 950
FORD 3000
MILLER 1300
14 rows selected.
--oracle的写法
SQL> SELECT ename,sal
2 FROM emp2 e,dept2 d
3 WHERE e.deptno=d.deptno;
ENAME SAL
-------------------- ----------
SMITH 800
ALLEN 1600
WARD 1250
JONES 2975
MARTIN 1250
BLAKE 2850
CLARK 2450
SCOTT 3000
KING 5000
TURNER 1500
ADAMS 1100
JAMES 950
FORD 3000
MILLER 1300
14 rows selected.
三、OUTER JOIN 外连接
外连接，又分为：
1.LEFT OUTER JOIN/LEFT JOIN（左外连接）
2.RIGHT OUTER JOIN/RIGHT JOIN（右外连接）
3.FULL OUTER JOIN/FULL JOIN(全外连接)
1.LEFT OUTER JOIN/LEFT JOIN（左外连接）
左外连接就是关联的两张或多张表中，根据关联条件，显示匹配的记录，左表中有的记录，但是右表中没有匹配上的，以空(null)显示。
LEFT OUTER JOIN也可以简写成LEFT JOIN，效果是一样的。
左外连接有SQL标准写法，也有oracle特有的写法。
SQL> SELECT ename,sal,dname
2 FROM emp2 e LEFT OUTER JOIN dept2 d
3 ON e.deptno=d.deptno;
ENAME SAL DNAME
-------------------- ---------- -------------
MILLER 1300 ACCOUNTING
KING 5000 ACCOUNTING
CLARK 2450 ACCOUNTING
FORD 3000 RESEARCH
ADAMS 1100 RESEARCH
SCOTT 3000 RESEARCH
JONES 2975 RESEARCH
SMITH 800 RESEARCH
JAMES 950 SALES
TURNER 1500 SALES
BLAKE 2850 SALES
MARTIN 1250 SALES
WARD 1250 SALES
ALLEN 1600 SALES
test 4000
15 rows selected.
--也可以去掉OUTER关键字，写成LEFT JOIN
SQL> SELECT ename,sal,dname
2 FROM emp2 e LEFT JOIN dept2 d
3 ON e.deptno=d.deptno;
ENAME SAL DNAME
-------------------- ---------- -------------
MILLER 1300 ACCOUNTING
KING 5000 ACCOUNTING
CLARK 2450 ACCOUNTING
FORD 3000 RESEARCH
ADAMS 1100 RESEARCH
SCOTT 3000 RESEARCH
JONES 2975 RESEARCH
SMITH 800 RESEARCH
JAMES 950 SALES
TURNER 1500 SALES
BLAKE 2850 SALES
MARTIN 1250 SALES
WARD 1250 SALES
ALLEN 1600 SALES
test 4000
15 rows selected.
--如果两张表的关联字段名相同，也可以使用USING子句
SQL> SELECT ename,sal
2 FROM emp2 e LEFT JOIN dept2 d
3 USING(deptno);
ENAME SAL DNAME
-------------------- ---------- -----------
MILLER 1300 ACCOUNTING
KING 5000 ACCOUNTING
CLARK 2450 ACCOUNTING
FORD 3000 RESEARCH
ADAMS 1100 RESEARCH
SCOTT 3000 RESEARCH
JONES 2975 RESEARCH
SMITH 800 RESEARCH
JAMES 950 SALES
TURNER 1500 SALES
BLAKE 2850 SALES
MARTIN 1250 SALES
WARD 1250 SALES
ALLEN 1600 SALES
test 4000
15 rows selected.
-oracle的写法
SQL> SELECT ename,sal,dname
2 FROM emp2 e,dept2 d
3 WHERE e.deptno=d.deptno(+);
ENAME SAL DNAME
-------------------- ---------- -----------
MILLER 1300 ACCOUNTING
KING 5000 ACCOUNTING
CLARK 2450 ACCOUNTING
FORD 3000 RESEARCH
ADAMS 1100 RESEARCH
SCOTT 3000 RESEARCH
JONES 2975 RESEARCH
SMITH 800 RESEARCH
JAMES 950 SALES
TURNER 1500 SALES
BLAKE 2850 SALES
MARTIN 1250 SALES
WARD 1250 SALES
ALLEN 1600 SALES
test 4000
15 rows selected.
2.RIGHT OUTER JOIN/RIGHT JOIN（右外连接）
右外连接就是关联的两张或多张表中，根据关联条件，显示匹配的记录。右表中有的记录，但是左表中没有匹配上的，以空(null)显示。
RIGHT OUTER JOIN也可以简写成RIGHT JOIN，效果是一样的。
右外连接有SQL标准写法，也有oracle特有的写法。
SQL> SELECT ename,sal,dname
2 FROM emp2 e RIGHT OUTER JOIN dept2 d
3 ON e.deptno=d.deptno;
E
NAME SAL DNAME
-------------------- ---------- ---------------
SMITH 800 RESEARCH
ALLEN 1600 SALES
WARD 1250 SALES
JONES 2975 RESEARCH
MARTIN 1250 SALES
BLAKE 2850 SALES
CLARK 2450 ACCOUNTING
SCOTT 3000 RESEARCH
KING 5000 ACCOUNTING
TURNER 1500 SALES
ADAMS 1100 RESEARCH
JAMES 950 SALES
FORD 3000 RESEARCH
MILLER 1300 ACCOUNTING
OPERATIONS
HR
16 rows selected.
--去掉OUTER关键字
SQL> SELECT ename,sal,dname
2 FROM emp2 e RIGHT JOIN dept2 d
3 ON e.deptno=d.deptno;
ENAME SAL DNAME
-------------------- ---------- ------------
SMITH 800 RESEARCH
ALLEN 1600 SALES
WARD 1250 SALES
JONES 2975 RESEARCH
MARTIN 1250 SALES
BLAKE 2850 SALES
CLARK 2450 ACCOUNTING
SCOTT 3000 RESEARCH
KING 5000 ACCOUNTING
TURNER 1500 SALES
ADAMS 1100 RESEARCH
JAMES 950 SALES
FORD 3000 RESEARCH
MILLER 1300 ACCOUNTING
OPERATIONS
HR
16 rows selected.
--如果两张表的关联字段名相同，也可以使用USING子句
ENAME SAL DNAME
-------------------- ---------- -------------
SMITH 800 RESEARCH
ALLEN 1600 SALES
WARD 1250 SALES
JONES 2975 RESEARCH
MARTIN 1250 SALES
BLAKE 2850 SALES
CLARK 2450 ACCOUNTING
SCOTT 3000 RESEARCH
KING 5000 ACCOUNTING
TURNER 1500 SALES
ADAMS 1100 RESEARCH
JAMES 950 SALES
FORD 3000 RESEARCH
MILLER 1300 ACCOUNTING
OPERATIONS
HR
16 rows selected.
--oracle的写法
SQL> SELECT ename,sal,dname
2 FROM emp2 e,dept2 d
3 WHERE e.deptno(+)=d.deptno;
ENAME SAL DNAME
-------------------- ---------- ------------
SMITH 800 RESEARCH
ALLEN 1600 SALES
WARD 1250 SALES
JONES 2975 RESEARCH
MARTIN 1250 SALES
BLAKE 2850 SALES
CLARK 2450 ACCOUNTING
SCOTT 3000 RESEARCH
KING 5000 ACCOUNTING
TURNER 1500 SALES
ADAMS 1100 RESEARCH
JAMES 950 SALES
FORD 3000 RESEARCH
MILLER 1300 ACCOUNTING
OPERATIONS
HR
16 rows selected.
3.FULL OUTER JOIN/FULL JOIN（全外连接）
全外连接就是关联的两张或多张表中，根据关联条件，显示所有匹配和不匹配的记录。
左表中有的记录，但是右表中没有匹配上的，以空(null)显示。右表中有的记录，但是左表中没有匹配上的，也以空(null)显示。
FULL OUTER JOIN也可以简写成FULL JOIN，效果是一样的。
全外连接只有SQL标准写法，没有oracle特有的写法。
SQL> SELECT ename,sal,dname
2 FROM emp2 e FULL OUTER JOIN dept2 d
3 ON e.deptno=d.deptno;
ENAME SAL DNAME
-------------------- ---------- -------------
MILLER 1300 ACCOUNTING
KING 5000 ACCOUNTING
CLARK 2450 ACCOUNTING
FORD 3000 RESEARCH
ADAMS 1100 RESEARCH
SCOTT 3000 RESEARCH
JONES 2975 RESEARCH
SMITH 800 RESEARCH
JAMES 950 SALES
TURNER 1500 SALES
BLAKE 2850 SALES
MARTIN 1250 SALES
WARD 1250 SALES
ALLEN 1600 SALES
test 4000
OPERATIONS
HR
17 rows selected.
SQL> SELECT ename,sal,dname
2 FROM emp2 e FULL JOIN dept2 d
3 ON e.deptno=d.deptno;
ENAME SAL DNAME
-------------------- ---------- ----------------
MILLER 1300 ACCOUNTING
KING 5000 ACCOUNTING
CLARK 2450 ACCOUNTING
FORD 3000 RESEARCH
ADAMS 1100 RESEARCH
SCOTT 3000 RESEARCH
JONES 2975 RESEARCH
SMITH 800 RESEARCH
JAMES 950 SALES
TURNER 1500 SALES
BLAKE 2850 SALES
MARTIN 1250 SALES
WARD 1250 SALES
ALLEN 1600 SALES
test 4000
OPERATIONS
HR
17 rows selected.
--如果两张表的关联字段名相同，也可以使用USING子句
SQL> SELECT ename,sal,dname
2 FROM emp2 e FULL JOIN dept2 d
3 USING(deptno);
ENAME SAL DNAME
-------------------- ---------- -------------
MILLER 1300 ACCOUNTING
KING 5000 ACCOUNTING
CLARK 2450 ACCOUNTING
FORD 3000 RESEARCH
ADAMS 1100 RESEARCH
SCOTT 3000 RESEARCH
JONES 2975 RESEARCH
SMITH 800 RESEARCH
JAMES 950 SALES
TURNER 1500 SALES
BLAKE 2850 SALES
MARTIN 1250 SALES
WARD 1250 SALES
ALLEN 1600 SALES
test 4000
OPERATIONS
HR
17 rows selected.
--没有oracle写法，下面这样是错误的。
SQL> SELECT ename,sal,dname
2 FROM emp2 e,dept2 d
3 WHERE e.deptno(+)=d.deptno(+);
WHERE e.deptno(+)=d.deptno(+)
*
ERROR at line 3:
ORA-01468: a predicate may reference only one outer-joined table
四、SELF JOIN 自连接
在上面的emp2表中,每一个员工都有自己的mgr(经理)，并且每一个经理自身也是公司的员工，自身也有自己的经理。现在需要将每一个员工自己的名字和经理的名字都找出来，应该怎么显示呢？
如果我们有两张这样的表分别教worker和mgr，那么我们就很好写SQL语句。
SELECT worker.name,Mgr.name
FROM worker,mgr
WHERE worker.id = mgr.id;
但现在我们只有一张表。你也许说我们现在在建一张表，把同样的数据拷贝过去不就可以了吗？是的，这样可以，但我们不会采用，因为这样就会很麻烦，而且数据严重冗余等等很多弊端。
这里有更好的方法，那就是自连接。
自连接，就是把一张表取两个别名，当做两张表来使用,自己和自己关联。
SQL> SELECT e.ename,e.sal,e2.ename manager_name
2 FROM emp2 e inner join emp2 e2
3 ON e.empno=e2.mgr;
ENAME SAL MANAGER_NAME
-------------------- ---------- -------------------
FORD 3000 SMITH
BLAKE 2850 ALLEN
BLAKE 2850 WARD
KING 5000 JONES
BLAKE 2850 MARTIN
KING 5000 BLAKE
KING 5000 CLARK
JONES 2975 SCOTT
BLAKE 2850 TURNER
SCOTT 3000 ADAMS
BLAKE 2850 JAMES
JONES 2975 FORD
CLARK 2450 MILLER
13 rows selected.
SQL> SELECT e.ename,e.sal,e2.ename manager_name
2 FROM emp2 e, emp2 e2
3 WHERE e.empno=e2.mgr;
ENAME SAL MANAGER_NAME
-------------------- ---------- -------------------
FORD 3000 SMITH
BLAKE 2850 ALLEN
BLAKE 2850 WARD
KING 5000 JONES
BLAKE 2850 MARTIN
KING 5000 BLAKE
KING 5000 CLARK
JONES 2975 SCOTT
BLAKE 2850 TURNER
SCOTT 3000 ADAMS
BLAKE 2850 JAMES
JONES 2975 FORD
CLARK 2450 MILLER
13 rows selected.
SQL> SELECT e.ename,e.sal,e2.ename manager_name
2 FROM emp2 e, emp2 e2
3 WHERE e.empno=e2.mgr(+);
ENAME SAL MANAGER_NAME
-------------------- ---------- -------------------
FORD 3000 SMITH
BLAKE 2850 ALLEN
BLAKE 2850 WARD
KING 5000 JONES
BLAKE 2850 MARTIN
KING 5000 BLAKE
KING 5000 CLARK
JONES 2975 SCOTT
BLAKE 2850 TURNER
SCOTT 3000 ADAMS
BLAKE 2850 JAMES
JONES 2975 FORD
CLARK 2450 MILLER
TURNER 1500
WARD 1250
test 4000
MARTIN 1250
ALLEN 1600
MILLER 1300
SMITH 800
ADAMS 1100
JAMES 950
22 rows selected.
SQL> SELECT e.ename,e.sal,e2.ename manager_name
2 FROM emp2 e, emp2 e2
3 WHERE e.empno(+)=e2.mgr;
ENAME SAL MANAGER_NAME
-------------------- ---------- -----------------
JONES 2975 FORD
JONES 2975 SCOTT
BLAKE 2850 JAMES
BLAKE 2850 TURNER
BLAKE 2850 MARTIN
BLAKE 2850 WARD
BLAKE 2850 ALLEN
CLARK 2450 MILLER
SCOTT 3000 ADAMS
KING 5000 CLARK
KING 5000 BLAKE
KING 5000 JONES
FORD 3000 SMITH
test
KING
15 rows selected.
SQL> SELECT e.ename,e.sal,e2.ename manager_name
2 FROM emp2 e FULL JOIN emp2 e2
3 ON e.empno=e2.mgr;
ENAME SAL MANAGER_NAME
-------------------- ---------- --------------------
FORD 3000 SMITH
BLAKE 2850 ALLEN
BLAKE 2850 WARD
KING 5000 JONES
BLAKE 2850 MARTIN
KING 5000 BLAKE
KING 5000 CLARK
JONES 2975 SCOTT
BLAKE 2850 TURNER
SCOTT 3000 ADAMS
BLAKE 2850 JAMES
JONES 2975 FORD
CLARK 2450 MILLER
TURNER 1500
WARD 1250
test 4000
MARTIN 1250
ALLEN 1600
MILLER 1300
SMITH 800
ADAMS 1100
JAMES 950
test
KING
24 rows selected.
五、总结
表连接分为：
1.CROSS JOIN：显示迪卡尔积。
2.INNERT JOIN:显示所有匹配的项。
3.OUTER JOIN
外连接又分为:
1) LEFT OUTER JOIN/LEFT JOIN：显示左表的所有项，右表没有匹配的项，则以null显示。
2) RIGHT OUTER JOIN/RIGHT JOIN：显示右表的所有项，左表没有匹配的项，则以null显示。
3) FULL OUTER JOIN/FULL JOIN：显示所有匹配和不匹配的项，左右两张表没有匹配的，都以null显示。
4.SELF JOIN:把一张表取两个别名，当做两张表来使用，自己和自己关联。
