MySQL Join详解
添加时间: 2006-4-21 6:50:05 作者: Mysql教程 阅读次数:73 来源: http://www.d9soft.com
还是先 Create table 吧
create table emp(
id int not null primary key,
name varchar(10)
);
create table emp_dept(
dept_id varchar(4) not null,
emp_id int not null,
emp_name varchar(10),
primary key (dept_id,emp_id));
insert into emp() values
(1,"Dennis-1"),
(2,"Dennis-2"),
(3,"Dennis-3"),
(4,"Dennis-4"),
(5,"Dennis-5"),
(6,"Dennis-6"),
(7,"Dennis-7"),
(8,"Dennis-8"),
(9,"Dennis-9"),
(10,"Dennis-10");
insert into emp_dept() values
("R&D",1,"Dennis-1"),
("DEv",2,"Dennis-2"),
("R&D",3,"Dennis-3"),
("Test",4,"Dennis-4"),
("Test",5,"Dennis-5");
>> left join
-------------
select a.id,a.name,b.dept_id
from emp a left join emp_dept b on (a.id=b.emp_id);
# 挑出左边的 table emp 中的所有资料,即使 emp_dept 中没有的资料也挑出来,没有的就用 NULL 来显示,
# 也即显示资料是以左边的 table emp 中的资料为基础
mysql> select a.id,a.name,b.dept_id
-> from emp a left join emp_dept b on (a.id=b.emp_id);
+----+-----------+---------+
id name dept_id
+----+-----------+---------+
1 Dennis-1 R&D
2 Dennis-2 DEv
3 Dennis-3 R&D
4 Dennis-4 Test
5 Dennis-5 Test
6 Dennis-6 NULL
7 Dennis-7 NULL
8 Dennis-8 NULL
9 Dennis-9 NULL
10 Dennis-10 NULL
+----+-----------+---------+
# 挑出 table emp 中有而 table emp_dept 中没有的人员资料
select a.id,a.name,b.dept_id
from emp a left join emp_dept b on (a.id=b.emp_id)
where b.dept_id IS NULL;
mysql> select a.id,a.name,b.dept_id
-> from emp a left join emp_dept b on (a.id=b.emp_id)
-> where b.dept_id IS NULL;
+----+-----------+---------+
id name dept_id
+----+-----------+---------+
6 Dennis-6 NULL
7 Dennis-7 NULL
8 Dennis-8 NULL
9 Dennis-9 NULL
10 Dennis-10 NULL
+----+-----------+---------+
# 把 table emp_dept 放在左边的情形(当然以 emp_dept 中的数据为基础来显示资料,emp 中比emp_dept 中多的资料也就不会显示出来了):
select a.id,a.name,b.dept_id
from emp_dept b left join emp a on (a.id=b.emp_id);
mysql> select a.id,a.name,b.dept_id
-> from emp_dept b left join emp a on (a.id=b.emp_id);
+------+----------+---------+
id name dept_id
+------+----------+---------+
2 Dennis-2 DEv
1 Dennis-1 R&D
3 Dennis-3 R&D
4 Dennis-4 Test
5 Dennis-5 Test
+------+----------+---------+
>> right join
---------------
select a.id,a.name,b.dept_id
from emp a right join emp_dept b on (a.id=b.emp_id);
# 挑资料时以右边 table emp_dept 中的资料为基础来显示资料
mysql> select a.id,a.name,b.dept_id
-> from emp a right join emp_dept b on (a.id=b.emp_id);
+------+----------+---------+
id name dept_id
+------+----------+---------+
2 Dennis-2 DEv
1 Dennis-1 R&D
3 Dennis-3 R&D
4 Dennis-4 Test
5 Dennis-5 Test
+------+----------+---------+
5 rows in set (0.00 sec)
# 我们再把 table 的位置交换一下,再用 right join 试试
select a.id,a.name,b.dept_id
from emp_dept b right join emp a on (a.id=b.emp_id);
mysql> select a.id,a.name,b.dept_id
-> from emp_dept b right join emp a on (a.id=b.emp_id);
+----+-----------+---------+
id name dept_id
+----+-----------+---------+
1 Dennis-1 R&D
2 Dennis-2 DEv
3 Dennis-3 R&D
4 Dennis-4 Test
5 Dennis-5 Test
6 Dennis-6 NULL
7 Dennis-7 NULL
8 Dennis-8 NULL
9 Dennis-9 NULL
10 Dennis-10 NULL
+----+-----------+---------+
# 是不是和 left join 一样了?
>> direct join
--------------
# 如果用right join 同不用 Join 直接挑资料是相同的,它等介於以下的指令
select a.id,a.name,b.dept_id
from emp a ,emp_dept b
where a.id=b.emp_id;
mysql> select a.id,a.name,b.dept_id
-> from emp a ,emp_dept b
-> where a.id=b.emp_id;
+----+----------+---------+
id name dept_id
+----+----------+---------+
2 Dennis-2 DEv
1 Dennis-1 R&D
3 Dennis-3 R&D
4 Dennis-4 Test
5 Dennis-5 Test
+----+----------+---------+
怎样,弄明白了吗?
Enjoy it!
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