左右连接
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)
数据库中实现两表*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看成集合有三个元素
test11看成集合有两个元素
test10*test11得到新集合是有六个元素
行 两个表行数相乘
列 两个表列数相加（可以重复）
多表查询时候列名重复的情况时候，需要明确指出取得是那个表
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 | 手机类型 | | 4 | 8 | htcN85原装充电器 | 1 | 手机类型 | | 3 | 8 | 诺基亚原装5800耳机 | 1 | 手机类型 | +----------+--------+--------------------+--------+----------+
技巧 ：创建一个同样结构的表 create table [新表名] like [旧表名]
create table minigoods like goods;
复制一部分表的内容
mysql> insert into minigoods -> select * from goods limit 3;
取得有意义的两个表的对应 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手机 | | 4 | 8 | htcN85原装充电器 | 8 | 耳机 | | 3 | 8 | 诺基亚原装5800耳机 | 8 | 耳机 | +----------+--------+--------------------+--------+----------+
3 rows in set (0.00 sec)
左连接语法
a表在左，不动
b表在右，动
a表和b表通过一个关系（自己设定），来筛选a所需要的b的行
a left join b on [条件] —-条件为真，取出b的行
a left join b on [条件] 结果集也可以看成表（假设为c表），可以对其再查询
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手机 | | 4 | htcN85原装充电器 | 耳机 | | 3 | 诺基亚原装5800耳机 | 耳机 | +----------+--------------------+----------+
[minigoods left join category on minigoods.cat_id=category.cat_id 看做c表]
验证：就是后面依然可以使用where 等等过滤条件
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原装充电器 | 耳机 | | 3 | 诺基亚原装5800耳机 | 耳机 | +----------+--------------------+----------+
2 rows in set (0.00 sec)
可以多张表左连接，就是把结果看成表
看做一张表
{{a left join b on [条件]} left join c on [条件]} 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手机 | | 4 | htcN85原装充电器 | 8 | 3G手机 | | 3 | 诺基亚原装5800耳机 | 8 | 3G手机 | | 5 | 索爱原装M2卡读卡器 | 11 | 3G手机 | +----------+--------------------+--------+----------+
左右连接的区别===================================================================================================
a left join b on 意思是查询的时候以a为基准查询
a right join b on 意思是查询的时候以b为基准查询
a left join b on 等同于 b right join a （都是以a为基准查询）
技巧：处于移植兼容性和理解上来说，最好一律使用左连接实现
create table boy( bname varchar(20), other char(1) )engine myisam charset utf8; insert into boy values ('屌丝','A'), ('李四','B'), ('王五','C'), ('高富帅','D'), ('郑七','E');
create table girl( gname varchar(20), other char(1) )engine myisam charset utf8; insert into girl values ('空姐','B'), ('大S','C'), ('阿娇','D'), ('张柏芝','D'), ('林黛玉','E'), ('宝钗','F');
取出所有男生的配偶（左连接）
select boy.*,girl.* from boy left join girl on boy.other=girl.other; +--------+-------+--------+-------+ | bname | other | gname | other | +--------+-------+--------+-------+ | 屌丝 | A | NULL | NULL | | 李四 | B | 空姐 | B | | 王五 | C | 大S | C | | 高富帅 | D | 阿娇 | D | | 高富帅 | D | 张柏芝 | D | | 郑七 | E | 林黛玉 | E | +--------+-------+--------+-------+
取出所有女生的配偶（左连接）
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 | | 阿娇 | D | 高富帅 | D | | 张柏芝 | D | 高富帅 | D | | 林黛玉 | E | 郑七 | E | | 宝钗 | F | NULL | NULL | +--------+-------+--------+-------+
取出所有女生的配偶（右连接，和上面左连接一致）
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 | | 阿娇 | D | 高富帅 | D | | 张柏芝 | D | 高富帅 | D | | 林黛玉 | E | 郑七 | E | | 宝钗 | F | NULL | NULL | +--------+-------+--------+-------+
内连接===================================================================================================
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 | | 阿娇 | D | 高富帅 | D | | 张柏芝 | D | 高富帅 | D | | 林黛玉 | E | 郑七 | E | +--------+-------+--------+-------+
内连接是左右连接的交集
（外连接是左右连接的并集，mysql不支持）可以使用用union实现
左连接应用===================================================================================================
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,'国安'), (3,'申花'), (4,'大连'); 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 | 国安 | | 3 | 申花 | | 4 | 大连 | +---------+-----------+
技巧 ：善与给表其别名
2006-06-21日期之后 把表中对战换成队名
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 | 国安 | 2:0 | 2006-05-21 | | 2 | 国安 | 3 | 申花 | 1:2 | 2006-06-21 | | 3 | 申花 | 1 | 恒大 | 2:5 | 2006-07-21 | | 4 | 大连 | 1 | 恒大 | 3:2 | 2006-08-21 | +--------------+-----------+---------------+-----------+--------------+------------+
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