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oracle中的left、right连接测试

原创 Linux操作系统 作者:haha牛 时间:2012-07-02 18:09:35 0 删除 编辑

create table mytest1(id number,name varchar2(20));
insert into mytest1 select 1,'a' from dual;
insert into mytest1 select 2,'b' from dual;
insert into mytest1 select 3,'c' from dual;
insert into mytest1 select null,'d' from dual;

create table mytest2(id number,name varchar2(20));
insert into mytest2 select 1,'A' from dual;
insert into mytest2 select 2,'B' from dual;
insert into mytest2 select 3,'C' from dual;
insert into mytest2 select null,'D' from dual;

create table mytest3(id number,name varchar2(20));
insert into mytest3 select 11,'a' from dual;
insert into mytest3 select 22,'b' from dual;
insert into mytest3 select 33,'c' from dual;
insert into mytest3 select null,'d' from dual;
commit;

======================================================

SQL> select * from mytest1 join mytest2 on(mytest1.id=mytest2.id);

        ID NAME                         ID NAME
---------- -------------------- ---------- --------------------
         1 a                             1 A
         2 b                             2 B
         3 c                             3 C

SQL> select * from mytest1,mytest2 where mytest1.id=mytest2.id;

        ID NAME                         ID NAME
---------- -------------------- ---------- --------------------
         1 a                             1 A
         2 b                             2 B
         3 c                             3 C

======================================================

SQL> select * from mytest1 right join mytest2 on(mytest1.id=mytest2.id);

        ID NAME                         ID NAME
---------- -------------------- ---------- --------------------
         1 a                             1 A
         2 b                             2 B
         3 c                             3 C
                                           D

SQL> select * from mytest1,mytest2 where mytest1.id(+)=mytest2.id;

        ID NAME                         ID NAME
---------- -------------------- ---------- --------------------
         1 a                             1 A
         2 b                             2 B
         3 c                             3 C
                                           D

======================================================

SQL> select * from mytest1 left join mytest2 on(mytest1.id=mytest2.id);

        ID NAME                         ID NAME
---------- -------------------- ---------- --------------------
         1 a                             1 A
         2 b                             2 B
         3 c                             3 C
           d

SQL> select * from mytest1,mytest2 where mytest1.id=mytest2.id(+);

        ID NAME                         ID NAME
---------- -------------------- ---------- --------------------
         1 a                             1 A
         2 b                             2 B
         3 c                             3 C
           d

======================================================

SQL> select * from mytest1 left join mytest2 on(mytest1.id=mytest2.id)
  2  left join mytest3 on(mytest1.name=mytest3.name);

        ID NAME                         ID NAME                         ID NAME
---------- -------------------- ---------- -------------------- ---------- --------------------
         1 a                             1 A                            11 a
         2 b                             2 B                            22 b
         3 c                             3 C                            33 c
           d                                                               d
SQL> select * from mytest1,mytest2,mytest3 where mytest1.id=mytest2.id(+) and mytest1.name=mytest3.name(+);

        ID NAME                         ID NAME                         ID NAME
---------- -------------------- ---------- -------------------- ---------- --------------------
         1 a                             1 A                            11 a
         2 b                             2 B                            22 b
         3 c                             3 C                            33 c
           d                                                               d

======================================================

SQL> select * from mytest1 left join mytest2 on(mytest1.id=mytest2.id and mytest2.name='B')
  2  left join mytest3 on(mytest1.name=mytest3.name);

        ID NAME                         ID NAME                         ID NAME
---------- -------------------- ---------- -------------------- ---------- --------------------
         1 a                                                            11 a
         2 b                             2 B                            22 b
         3 c                                                            33 c
           d                                                               d

SQL> select * from mytest1,(select * from mytest2 where name='B') mytt2,mytest3
  2  where mytest1.id=mytt2.id(+)
  3  and mytest1.name=mytest3.name(+);

        ID NAME                         ID NAME                         ID NAME
---------- -------------------- ---------- -------------------- ---------- --------------------
         1 a                                                            11 a
         2 b                             2 B                            22 b
         3 c                                                            33 c
           d                                                               d

SQL> select * from mytest1,mytest2,mytest3
  2  where mytest1.id=mytest2.id(+)
  3  and mytest2.name='B'
  4  and mytest1.name=mytest3.name(+);

        ID NAME                         ID NAME                         ID NAME
---------- -------------------- ---------- -------------------- ---------- --------------------
         2 b                             2 B                            22 b

======================================================

数据库在通过连接两张或多张表来返回记录时,都会生成一张中间的临时表,然后再将这张临时表返回给用户。

在使用left jion时,on和where条件的区别如下:

1、 on条件是在生成临时表时使用的条件,它不管on中的条件是否为真,都会返回左边表中的记录。

2、where条件是在临时表生成好后,再对临时表进行过滤的条件。这时已经没有left join的含义(必须返回左边表的记录)了,条件不为真的

就全部过滤掉。

假设有两张表:

表1 mytest1:

        ID NAME
---------- --------------------
         1 a
         2 b
         3 c
           d

表2 mytest2:

        ID NAME
---------- --------------------
         1 A
         2 B
         3 C
           D


两条SQL:
1、select * from mytest1 left join mytest2 on (mytest1.id=mytest2.id) where mytest2.name='B';
2、select * form. mytest1 left join mytest2 on (mytest1.id = mytest2.id and mytest2.name='B');

第一条SQL的过程:

1、中间表
on条件:
mytest1.id=mytest2.id

        ID NAME                         ID NAME
---------- -------------------- ---------- --------------------
         1 a                             1 A
         2 b                             2 B
         3 c                             3 C
           d

2、再对中间表过滤
where 条件:
mytest2.name='B';

        ID NAME                         ID NAME
---------- -------------------- ---------- --------------------
         2 b                             2 B


第二条SQL的过程:

1、中间表
on条件:
mytest1.id=mytest2.id and mytest2.name='B'

        ID NAME
---------- --------------------
         2 B

2、对结果集进行连接.
(条件不为真也会返回左表中的记录)

        ID NAME                         ID NAME
---------- -------------------- ---------- --------------------
         2 b                             2 B
            d
         3 c
         1 a


其实以上结果的关键原因就是left join,right join,full join的特殊性,不管on上的条件是否为真都会返回left或right表中的记录,

full则具有left和right的特性的并集。 而inner jion没这个特殊性,则条件放在on中和where中,返回的结果集是相同的。

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