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!
作者:DennisLan
[MySql]MySQL Join详解
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