全相乘方法（效率很低）
mysql> select * from test10; +------+-------+ | id | sname | +------+-------+ | 1 | 云彩 | | 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 | 云彩 | 95 | 猴子 | | 1 | 云彩 | 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 兩個表行數相乘
列 兩個表列數相加（可以重復）
多表查詢時候列名重復的情況時候，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 [舊表名]
create table minigoods like goods;
復制一部分表的內容
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表通過一個關系（自己設定），來篩選a所需要的b的行
a left join b on [條件] ----條件為真，取出b的行
a left join b on [條件] 結果集也可以看成表（假設為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：就是后面依然可以使用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為基準查詢
a right join b on 意思是查詢的時候以b為基準查詢
a left join b on 等同于 b right join a （都是以a為基準查詢）
技巧：處于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并集，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,'恒大'), (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 | 恒大 | | 2 | 國安 | | 3 | 申花 | | 4 | 大連 | +---------+-----------+
技巧 ：善與給表其別名
2006-06-21日期之后 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 | 恒大 | 2 | 國安 | 2:0 | 2006-05-21 | | 2 | 國安 | 3 | 申花 | 1:2 | 2006-06-21 | | 3 | 申花 | 1 | 恒大 | 2:5 | 2006-07-21 | | 4 | 大連 | 1 | 恒大 | 3:2 | 2006-08-21 | +--------------+-----------+---------------+-----------+--------------+------------+
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