左右連接
全相乘方法（效率很低）
mysql> select * from test10;
+------+-------+
| id | sname |
+------+-------+
| 1 | 雲彩 |
| 2 | 月亮 |
| 3 | 星星 |
+------+-------+
3 rows in set (0.00 sec)
mysql> select * from test11;
+--------+-------+
| cat_id | cname |
+--------+-------+
| 95 | 猴子 |
| 96 | 老虎 |
+--------+-------+
2 rows in set (0.00 sec)
數據庫中實現兩表*的效果
mysql> select * from test10,test11;
+------+-------+--------+-------+
| id | sname | cat_id | cname |
+------+-------+--------+-------+
| 1 | 雲彩 | 95 | 猴子 |
| 1 | 雲彩 | 96 | 老虎 |
| 2 | 月亮 | 95 | 猴子 |
| 2 | 月亮 | 96 | 老虎 |
| 3 | 星星 | 95 | 猴子 |
| 3 | 星星 | 96 | 老虎 |
+------+-------+--------+-------+
6 rows in set (0.03 sec)
分析：
test10看成集合有三個元素
test11看成集合有兩個元素
test10*test11得到新集合是有六個元素
行 兩個錶行數相乘
列 兩個表列數相加（可以重複）
多表查詢時候列名重複的情況時候，需要明確指出取得是那個表
mysql> select goods_id,minigoods.cat_id,goods_name,category.cat_id,cat_name from minigoods,category limit 3;
+----------+--------+--------------------+--------+----------+
| goods_id | cat_id | goods_name | cat_id | cat_name |
+----------+--------+--------------------+--------+----------+
| 1 | 4 | KD876 | 1 | 手機類型 |
| 4 | 8 | htcN85原裝充電器 | 1 | 手機類型 |
| 3 | 8 | 諾基亞原裝5800耳機 | 1 | 手機類型 |
+----------+--------+--------------------+--------+----------+
技巧 ：創建一個同樣結構的表 create table [新表名] like [舊錶名]
create table minigoods like goods;
複製一部分表的內容
mysql> insert into minigoods
-> select * from goods limit 3;
取得有意義的兩個表的對應 minigoods.cat_id=category.cat_id
mysql> select goods_id,minigoods.cat_id,goods_name,category.cat_id,cat_name from minigoods,category where minigoods.cat_id=category.cat_id ;
+----------+--------+--------------------+--------+----------+
| goods_id | cat_id | goods_name | cat_id | cat_name |
+----------+--------+--------------------+--------+----------+
| 1 | 4 | KD876 | 4 | 3G手機 |
| 4 | 8 | htcN85原裝充電器 | 8 | 耳機 |
| 3 | 8 | 諾基亞原裝5800耳機 | 8 | 耳機 |
+----------+--------+--------------------+--------+----------+
3 rows in set (0.00 sec)
左連接語法
a表在左，不動
b表在右，動
a表和b表通過一個關係（自己設定），來篩選a所需要的b的行
a left join b on [條件] ----條件爲真，取出b的行
a left join b on [條件] 結果集也可以看成表（假設爲c表），可以對其再查詢
mysql> select goods_id,goods_name,cat_name
-> from
-> (minigoods left join category on minigoods.cat_id=category.cat_id);
+----------+--------------------+----------+
| goods_id | goods_name | cat_name |
+----------+--------------------+----------+
| 1 | KD876 | 3G手機 |
| 4 | htcN85原裝充電器 | 耳機 |
| 3 | 諾基亞原裝5800耳機 | 耳機 |
+----------+--------------------+----------+
[minigoods left join category on minigoods.cat_id=category.cat_id 看做c表]
驗證：就是後面依然可以使用where 等等過濾條件
mysql> select goods_id,goods_name,cat_name from
(minigoods left join category on minigoods.cat_id=category.cat_id )
where 1 order by goods_id desc limit 2;
+----------+--------------------+----------+
| goods_id | goods_name | cat_name |
+----------+--------------------+----------+
| 4 | htcN85原裝充電器 | 耳機 |
| 3 | 諾基亞原裝5800耳機 | 耳機 |
+----------+--------------------+----------+
2 rows in set (0.00 sec)
可以多張表左連接，就是把結果看成表
看做一張表
{{a left join b on [條件]} left join c on [條件]}
mysql> select goods.goods_id,goods.goods_name,goods.cat_id,cat_name
-> from
-> minigoods left join category on minigoods.cat_id=category.cat_id
-> left join goods on minigoods.cat_id=4 limit 4;
+----------+--------------------+--------+----------+
| goods_id | goods_name | cat_id | cat_name |
+----------+--------------------+--------+----------+
| 1 | KD876 | 4 | 3G手機 |
| 4 | htcN85原裝充電器 | 8 | 3G手機 |
| 3 | 諾基亞原裝5800耳機 | 8 | 3G手機 |
| 5 | 索愛原裝M2卡讀卡器 | 11 | 3G手機 |
+----------+--------------------+--------+----------+
左右連接的區別===================================================================================================
a left join b on 意思是查詢的時候以a爲基準查詢
a right join b on 意思是查詢的時候以b爲基準查詢
a left join b on 等同於 b right join a （都是以a爲基準查詢）
技巧：處於移植兼容性和理解上來說，最好一律使用左連接實現
create table boy(
bname varchar(20),
other char(1)
)engine myisam charset utf8;
insert into boy
values
('屌絲','A'),
('李四','B'),
('王五','C'),
('高富帥','D'),
('鄭七','E');
create table girl(
gname varchar(20),
other char(1)
)engine myisam charset utf8;
insert into girl
values
('空姐','B'),
('大S','C'),
('阿嬌','D'),
('張柏芝','D'),
('林黛玉','E'),
('寶釵','F');
取出所有男生的配偶（左連接）
select boy.*,girl.* from
boy left join girl on
boy.other=girl.other;
+--------+-------+--------+-------+
| bname | other | gname | other |
+--------+-------+--------+-------+
| 屌絲 | A | NULL | NULL |
| 李四 | B | 空姐 | B |
| 王五 | C | 大S | C |
| 高富帥 | D | 阿嬌 | D |
| 高富帥 | D | 張柏芝 | D |
| 鄭七 | E | 林黛玉 | E |
+--------+-------+--------+-------+
取出所有女生的配偶（左連接）
mysql> select girl.*,boy.* from
-> girl left join boy on
-> boy.other=girl.other;
+--------+-------+--------+-------+
| gname | other | bname | other |
+--------+-------+--------+-------+
| 空姐 | B | 李四 | B |
| 大S | C | 王五 | C |
| 阿嬌 | D | 高富帥 | D |
| 張柏芝 | D | 高富帥 | D |
| 林黛玉 | E | 鄭七 | E |
| 寶釵 | F | NULL | NULL |
+--------+-------+--------+-------+
取出所有女生的配偶（右連接，和上面左連接一致）
mysql> select girl.* ,boy.* from
-> boy right join girl on
-> boy.other=girl.other;
+--------+-------+--------+-------+
| gname | other | bname | other |
+--------+-------+--------+-------+
| 空姐 | B | 李四 | B |
| 大S | C | 王五 | C |
| 阿嬌 | D | 高富帥 | D |
| 張柏芝 | D | 高富帥 | D |
| 林黛玉 | E | 鄭七 | E |
| 寶釵 | F | NULL | NULL |
+--------+-------+--------+-------+
內連接===================================================================================================
取出有配偶的
select girl.*,boy.* from
boy inner join girl on
boy.other=girl.other;
+--------+-------+--------+-------+
| gname | other | bname | other |
+--------+-------+--------+-------+
| 空姐 | B | 李四 | B |
| 大S | C | 王五 | C |
| 阿嬌 | D | 高富帥 | D |
| 張柏芝 | D | 高富帥 | D |
| 林黛玉 | E | 鄭七 | E |
+--------+-------+--------+-------+
內連接是左右連接的交集
（外連接是左右連接的並集，mysql不支持）可以使用用union實現
左連接應用===================================================================================================
create table match_t(
match_id int primary key auto_increment,
host_team_id int,
guest_team_id int,
match_result varchar(20),
match_time date
)engine myisam charset utf8;
insert into match_t
values
(1,1,2,'2:0','2006-05-21'),
(2,2,3,'1:2','2006-06-21'),
(3,3,1,'2:5','2006-07-21'),
(4,1,1,'3:2','2006-08-21');
create table team_t(
team_id int primary key auto_increment,
team_name varchar(20)
)engine myisam charset utf8;
insert into team_t
values
(1,'恆大'),
(2,'國安'),
(3,'申花'),
(4,'大連');
mysql> select * from match_t;
+----------+--------------+---------------+--------------+------------+
| match_id | host_team_id | guest_team_id | match_result | match_time |
+----------+--------------+---------------+--------------+------------+
| 1 | 1 | 2 | 2:0 | 2006-05-21 |
| 2 | 2 | 3 | 1:2 | 2006-06-21 |
| 3 | 3 | 1 | 2:5 | 2006-07-21 |
| 4 | 4 | 1 | 3:2 | 2006-08-21 |
+----------+--------------+---------------+--------------+------------+
mysql> select * from team_t;
+---------+-----------+
| team_id | team_name |
+---------+-----------+
| 1 | 恆大 |
| 2 | 國安 |
| 3 | 申花 |
| 4 | 大連 |
+---------+-----------+
技巧 ：善與給表其別名
2006-06-21日期之後 把表中對戰換成隊名
select host_t.team_name,guest_t.team_name,match_result,match_time from
match_t left join (team_t as host_t) on match_t.host_team_id=host_t.team_id
left join (team_t as guest_t) on match_t.guest_team_id=guest_t.team_id
where match_time>='2006-06-21';
+--------------+-----------+---------------+-----------+--------------+------------+
| host_team_id | team_name | guest_team_id | team_name | match_result | match_time |
+--------------+-----------+---------------+-----------+--------------+------------+
| 1 | 恆大 | 2 | 國安 | 2:0 | 2006-05-21 |
| 2 | 國安 | 3 | 申花 | 1:2 | 2006-06-21 |
| 3 | 申花 | 1 | 恆大 | 2:5 | 2006-07-21 |
| 4 | 大連 | 1 | 恆大 | 3:2 | 2006-08-21 |
+--------------+-----------+---------------+-----------+--------------+------------+