MySQL Join 詳解_MySql_学习笔记★闵涛★计算机学习电脑编程软硬件技巧


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MySQL Join 詳解

作者：闵涛文章来源：闵涛的学习笔记点击数：742 更新时间：2009/4/22 20:56:42

MySQL Join 詳解

(C) by Dennis Dll 2004.1.29

還是先 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);

# 挑出左邊的 table emp 中的所有資料,即使 emp_dept 中沒有的資料也挑出來,沒有的就用 NULL 來顯示,
# 也即顯示資料是以左邊的 table emp 中的資料為基礎

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 中沒有的人員資料
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;

# 把 table emp_dept 放在左邊的情形(當然以 emp_dept 中的數據為基礎來顯示資料,emp 中比emp_dept 中多的資料也就不會顯示出來了):

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);
# 挑資料時以右邊 table emp_dept 中的資料為基礎來顯示資料

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)

# 我們再把 table 的位置交換一下,再用 right join 試試

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 一樣了?

>> direct join
--------------
# 如果用right join 同不用 Join 直接挑資料是相同的,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    |
+----+----------+---------+

怎樣,弄明白了嗎?

Enjoy it!