为了解释这些问题,假设我们有一个表 t1
num | name -----+------ 1 | a 2 | b 3 | c
和 t2
num | value -----+------- 1 | xxx 3 | yyy 5 | zzz
然后我们用不同的连接方式可以获得各种结果:
=> SELECT * FROM t1 CROSS JOIN t2; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 1 | a | 3 | yyy 1 | a | 5 | zzz 2 | b | 1 | xxx 2 | b | 3 | yyy 2 | b | 5 | zzz 3 | c | 1 | xxx 3 | c | 3 | yyy 3 | c | 5 | zzz (9 rows) => SELECT * FROM t1 INNER JOIN t2 ON t1.num = t2.num; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 3 | c | 3 | yyy (2 rows) => SELECT * FROM t1 INNER JOIN t2 USING (num); num | name | value -----+------+------- 1 | a | xxx 3 | c | yyy (2 rows) => SELECT * FROM t1 NATURAL INNER JOIN t2; num | name | value -----+------+------- 1 | a | xxx 3 | c | yyy (2 rows) => SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 2 | b | | 3 | c | 3 | yyy (3 rows) => SELECT * FROM t1 LEFT JOIN t2 USING (num); num | name | value -----+------+------- 1 | a | xxx 2 | b | 3 | c | yyy (3 rows) => SELECT * FROM t1 RIGHT JOIN t2 ON t1.num = t2.num; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 3 | c | 3 | yyy | | 5 | zzz (3 rows) => SELECT * FROM t1 FULL JOIN t2 ON t1.num = t2.num; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 2 | b | | 3 | c | 3 | yyy | | 5 | zzz (4 rows)
用 ON 声明的连接条件也可以包含与连接不直接相关 的条件。这种功能可能对某些查询很有用,但是需要我们仔细想想。 比如:
=> SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num AND t2.value = 'xxx'; num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 2 | b | | 3 | c | | (3 rows)
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我们以Oracle自带的表来做例子
主要两张表:dept、emp
一个是宠物,一个是员工表结构如下:
emp
name | null? | Type |
Empno | not null | number(4) |
ename | varchar2(10) | |
job | varchar2(9) | |
mgr | number(4) | |
hiredate | date | |
sal | number(7,2) | |
comm | number(7,2) | |
deptno | number(2) |
dept
这两张表 dept是主表 emp是子表,关联的列是deptno
name | null? | Type |
deptno | not null | number(2) |
dname | varchar2(14) | |
log | varchar2(13) |
这两张表 dept是主表 emp是子表,关联的列是deptno
dept表现有数据
emp表现有数据
inner join 意思是内连接 把匹配的信息全部查出来
SQL>select e.empno,e.ename,e.job,d.deptno,d.dname
from emp e inner join dept d on e.deptno=d.deptno order by e.empno;
from emp e inner join dept d on e.deptno=d.deptno order by e.empno;
查询的结果:
left join 左连接 意思是包含左边表所有记录,右边所有的匹配的记录,如果没有则用空补齐
SQL>select e.empno,e.ename,e.job,d.deptno,d.dname
from emp e left join dept d on e.deptno=d.deptno order by e.empno;
from emp e left join dept d on e.deptno=d.deptno order by e.empno;
解释SQL :这条SQL语句左边是EMP表 左连接结果查询出EMP所有的记录,然后根据左边表匹配出右边表DEPT所有的记录
查询结果如下:
right join 右连接 意思是包括右边表所有记录,匹配左边表的记录,如果没有则以空补齐
SQL>select
e.empno,e.ename,e.job,d.deptno,d.dname
from emp e right join dept d on e.deptno=d.deptno order by e.empno;
from emp e right join dept d on e.deptno=d.deptno order by e.empno;
解释SQL:这条SQL语句EMP在左边,DEPT在右边,然后我们采用右连接,就查出右边表所有的数据
查询结构:
full join 全连接 意思是左右表所有的记录全部显示出来
SQL>select
e.empno,e.ename,e.job,d.deptno,d.dname
from emp e full join dept d on e.deptno=d.deptno order by e.empno;
from emp e full join dept d on e.deptno=d.deptno order by e.empno;
解释SQL:这条SQL语句采用全连接,查询出左表和右表所有的值出来
查询结果:
介绍Oracle +连接方式及说明
+在左边 为右连接
SQL>select e.empno,e.ename,e.job,d.deptno,d.dname
from emp e,dept d where e.deptno(+)=d.deptno order by e.empno;
from emp e,dept d where e.deptno(+)=d.deptno order by e.empno;
查询结果:
+在右边 为左连接
SQL>select e.empno,e.ename,e.job,d.deptno,d.dname from emp e,dept d where e.deptno=d.deptno(+) order by e.empno;
查询结果: