??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??的 table emp 中的所有儋料,即使 emp_dept 中?]有的儋料也挑出?憩?]有的就用 NULL ?盹@示,
# 也即锢示儋料是以左??的 table emp 中的儋料?榛?盗

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?]有的人?T儋料
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??的情形(??然以 emp_dept 中的?????榛?盗?盹@示儋料,emp 中比emp_dept 中多的儋料也就不??锢示出?砹拴:

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);
# 挑儋料?r以右?? table emp_dept 中的儋料?榛?盗?盹@示儋料

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??再把 table 的位置交?Q一下,再用 right join ????

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?恿丝

>> direct join
--------------
# 如果用right join 同不用 Join 直接挑儋料是相同的,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?蝇弄明白了?峥