SQL中join on條件和where條件的區別

SQL中join on條件和where條件的區別

簡單的說join on條件是對臨時表橫向（列級別）或縱向（行級別）數據的增加；而where條件則是對臨時表中縱向（行級別）數據的過濾。

用代碼說話

####################
## 對數據庫表的初始化
####################
create table tbl_user (
    id int auto_increment,
    name varchar(32) not null,
    group_id int not null,

    primary key(id),
    index (group_id)
) comment='用戶表';

create table tbl_group (
    id int auto_increment,
    name varchar(32) not null,

    primary key(id)
)comment='分組表';

insert into tbl_group (name) values('group');
insert into tbl_user (name, group_id) values('user01', 1);
insert into tbl_user (name, group_id) values('user02', 1);

###########################################################
## ① 通過SQL驗證join on條件可以對臨時表進行橫向（列級別）數據的增加
###########################################################
select u.id as u_id, u.name as u_name, u.group_id as u_group_id, g.id as g_id, g.name as g_name
from tbl_user as u
    left join tbl_group as g
        on g.id = u.group_id;

+------+--------+------------+------+--------+
| u_id | u_name | u_group_id | g_id | g_name |
+------+--------+------------+------+--------+
|    1 | user01 |          1 |    1 | group  |
|    2 | user02 |          1 |    1 | group  |
+------+--------+------------+------+--------+

#############################################################
## ② 通過SQL查詢驗證join on條件可以對臨時表進行縱向（行級別）數據增加
#############################################################
select g.id as g_id, g.name as g_name, u.id as u_id, u.name as u_name, u.group_id as u_group_id
from tbl_group as g
    left join tbl_user as u
        on g.id = u.group_id;

+------+--------+------+--------+------------+
| g_id | g_name | u_id | u_name | u_group_id |
+------+--------+------+--------+------------+
|    1 | group  |    1 | user01 |          1 |
|    1 | group  |    2 | user02 |          1 |
+------+--------+------+--------+------------+

######################################################
## ③ 通過SQL查詢驗證where條件對臨時表縱向（行級別）數據的過濾
######################################################
select g.id as g_id, g.name as g_name, u.id as u_id, u.name as u_name, u.group_id as u_group_id
from tbl_group as g
    left join tbl_user as u
        on g.id = u.group_id
where u.id = 1;

+------+--------+------+--------+------------+
| g_id | g_name | u_id | u_name | u_group_id |
+------+--------+------+--------+------------+
|    1 | group  |    1 | user01 |          1 |
+------+--------+------+--------+------------+