Mysql----Join用法(Inner join,Left join,Right join, Cross join, Union模拟Full join)及---性能优化
来源:互联网 发布:淘宝怎么判定定制产品 编辑:程序博客网 时间:2024/05/17 18:16
前期数据准备
CREATE TABLE atable(
aID int( 1 ) AUTO_INCREMENT PRIMARY KEY ,
aNum char( 20 ));
CREATE TABLE btable(
bID int( 1 ) NOT NULL AUTO_INCREMENT PRIMARY KEY ,
bName char( 20 ) );
INSERT INTO atable
VALUES ( 1, 'a20050111' ) , ( 2, 'a20050112' ) , ( 3, 'a20050113' ) , ( 4, 'a20050114' ) , ( 5, 'a20050115' ) ;
INSERT INTO btable
VALUES ( 1, ' 2006032401' ) , ( 2, '2006032402' ) , ( 3, '2006032403' ) , ( 4, '2006032404' ) , ( 8, '2006032408' ) ;
-------------------------------------------------------------------------------------------
atable:左表;btable:右表。
JOIN 按照功能大致分为如下三类:
1).inner join(内连接,或等值连接):取得两个表中存在连接匹配关系的记录。
2).left join(左连接):取得左表(atable)完全记录,即是右表(btable)并无对应匹配记录。
3).right join(右连接):与 LEFT JOIN 相反,取得右表(btable)完全记录,即是左表(atable)并无匹配对应记录。
注意:mysql不支持Full join,不过可以通过 union 关键字来合并 left join 与 right join来模拟full join.
一、Inner join
内连接,也叫等值连接,inner join产生同时符合A和B的一组数据。
接下来给出一个列子用于解释下面几种分类。如下两个表(A,B)
mysql> select * from atable inner join btable on atable.aid=btable.bid;+-----+-----------+-----+-------------+| aID | aNum | bID | bName |+-----+-----------+-----+-------------+| 1 | a20050111 | 1 | 2006032401 || 2 | a20050112 | 2 | 2006032402 || 3 | a20050113 | 3 | 2006032403 || 4 | a20050114 | 4 | 2006032404 |+-----+-----------+-----+-------------+
二、Left join
left join,(或left outer join:在Mysql中两者等价,推荐使用left join.)左连接从左表(A)产生一套完整的记录,与匹配的记录(右表(B)) .如果没有匹配,右侧将包含null。
mysql> select * from atable left join btable on atable.aid=btable.bid;+-----+-----------+------+-------------+| aID | aNum | bID | bName |+-----+-----------+------+-------------+| 1 | a20050111 | 1 | 2006032401 || 2 | a20050112 | 2 | 2006032402 || 3 | a20050113 | 3 | 2006032403 || 4 | a20050114 | 4 | 2006032404 || 5 | a20050115 | NULL | NULL |+-----+-----------+------+-------------+
------------------------------------------------------------------------------------------------------------
2).如果想只从左表(A)中产生一套记录,但不包含右表(B)的记录,可以通过设置where语句来执行,如下
mysql> select * from atable left join btable on atable.aid=btable.bid -> where atable.aid is null or btable.bid is null;+-----+-----------+------+-------+| aID | aNum | bID | bName |+-----+-----------+------+-------+| 5 | a20050115 | NULL | NULL |+-----+-----------+------+-------+
-----------------------------------------------------------------------------------------
同理,还可以模拟inner join. 如下:
mysql> select * from atable left join btable on atable.aid=btable.bid where atable.aid is not null and btable.bid is not null;+-----+-----------+------+-------------+| aID | aNum | bID | bName |+-----+-----------+------+-------------+| 1 | a20050111 | 1 | 2006032401 || 2 | a20050112 | 2 | 2006032402 || 3 | a20050113 | 3 | 2006032403 || 4 | a20050114 | 4 | 2006032404 |+-----+-----------+------+-------------+------------------------------------------------------------------------------------------
三、Right join
同Left joinmysql> select * from atable right join btable on atable.aid=btable.bid;+------+-----------+-----+-------------+| aID | aNum | bID | bName |+------+-----------+-----+-------------+| 1 | a20050111 | 1 | 2006032401 || 2 | a20050112 | 2 | 2006032402 || 3 | a20050113 | 3 | 2006032403 || 4 | a20050114 | 4 | 2006032404 || NULL | NULL | 8 | 2006032408 |+------+-----------+-----+-------------+
四、差集
mysql> select * from atable left join btable on atable.aid=btable.bid -> where btable.bid is null -> union -> select * from atable right join btable on atable.aid=btable.bid -> where atable.aid is null;+------+-----------+------+------------+| aID | aNum | bID | bName |+------+-----------+------+------------+| 5 | a20050115 | NULL | NULL || NULL | NULL | 8 | 2006032408 |+------+-----------+------+------------+
-----------------------------------------------------------------------------------
五.Cross join
交叉连接,得到的结果是两个表的乘积,即笛卡尔积
笛卡尔(Descartes)乘积又叫直积。假设集合A={a,b},集合B={0,1,2},则两个集合的笛卡尔积为{(a,0),(a,1),(a,2),(b,0),(b,1), (b,2)}。可以扩展到多个集合的情况。类似的例子有,如果A表示某学校学生的集合,B表示该学校所有课程的集合,则A与B的笛卡尔积表示所有可能的选课情况。
mysql> select * from atable cross join btable;+-----+-----------+-----+-------------+| aID | aNum | bID | bName |+-----+-----------+-----+-------------+| 1 | a20050111 | 1 | 2006032401 || 2 | a20050112 | 1 | 2006032401 || 3 | a20050113 | 1 | 2006032401 || 4 | a20050114 | 1 | 2006032401 || 5 | a20050115 | 1 | 2006032401 || 1 | a20050111 | 2 | 2006032402 || 2 | a20050112 | 2 | 2006032402 || 3 | a20050113 | 2 | 2006032402 || 4 | a20050114 | 2 | 2006032402 || 5 | a20050115 | 2 | 2006032402 || 1 | a20050111 | 3 | 2006032403 || 2 | a20050112 | 3 | 2006032403 || 3 | a20050113 | 3 | 2006032403 || 4 | a20050114 | 3 | 2006032403 || 5 | a20050115 | 3 | 2006032403 || 1 | a20050111 | 4 | 2006032404 || 2 | a20050112 | 4 | 2006032404 || 3 | a20050113 | 4 | 2006032404 || 4 | a20050114 | 4 | 2006032404 || 5 | a20050115 | 4 | 2006032404 || 1 | a20050111 | 8 | 2006032408 || 2 | a20050112 | 8 | 2006032408 || 3 | a20050113 | 8 | 2006032408 || 4 | a20050114 | 8 | 2006032408 || 5 | a20050115 | 8 | 2006032408 |+-----+-----------+-----+-------------+25 rows in set (0.00 sec) <pre><code class="hljs cs"><span class="hljs-function">#再执行:mysql> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> A inner <span class="hljs-keyword">join</span> B</span>; 试一试 (与上面的结果一样)<span class="hljs-meta">#在执行mysql> select * from A cross join B on A.name = B.name; 试一试</span></code>实际上,在 MySQL 中(仅限于 MySQL) CROSS JOIN 与 INNER JOIN 的表现是一样的,在不指定 ON 条件得到的结果都是笛卡尔积,反之取得两个表完全匹配的结果。 inner join 与 cross join 可以省略 inner 或 cross关键字,因此下面的 SQL 效果是一样的:
... FROM table1 INNER JOIN table2... FROM table1 CROSS JOIN table2... FROM table1 JOIN table2
六.union实现Full join
全连接产生的所有记录(双方匹配记录)在表A和表B。如果没有匹配,则对面将包含null。与差集类似。
mysql> select * from atable left join btable on atable.aid=btable.bid -> union -> select * from atable right join btable on atable.aid=btable.bid;+------+-----------+------+-------------+| aID | aNum | bID | bName |+------+-----------+------+-------------+| 1 | a20050111 | 1 | 2006032401 || 2 | a20050112 | 2 | 2006032402 || 3 | a20050113 | 3 | 2006032403 || 4 | a20050114 | 4 | 2006032404 || 5 | a20050115 | NULL | NULL || NULL | NULL | 8 | 2006032408 |+------+-----------+------+-------------+
--------------------------------------------------------------------------------------------------------
七.性能优化
1.显示(explicit) inner join VS 隐式(implicit) inner join
select * fromtable a inner join table bon a.id = b.id;VS
select a.*, b.*from table a, table bwhere a.id = b.id;数据库中比较(10w数据)得之,它们用时几乎相同,第一个是显示的inner join,后一个是隐式的inner join。
2.left join/right join VS inner join
尽量用inner join.避免 left join 和 null.
在使用left join(或right join)时,应该清楚的知道以下几点:
(1). on与 where的执行顺序
ON 条件(“A LEFT JOIN B ON 条件表达式”中的ON)用来决定如何从 B 表中检索数据行。如果 B 表中没有任何一行数据匹配 ON 的条件,将会额外生成一行所有列为 NULL 的数据,在匹配阶段 WHERE 子句的条件都不会被使用。仅在匹配阶段完成以后,WHERE 子句条件才会被使用。它将从匹配阶段产生的数据中检索过滤。
所以我们要注意:在使用Left (right) join的时候,一定要在先给出尽可能多的匹配满足条件,减少Where的执行。如:
select * from Ainner join B on B.name = A.nameleft join C on C.name = B.nameleft join D on D.id = C.idwhere C.status>1 and D.status=1;下面这种写法更省时
select * from Ainner join B on B.name = A.nameleft join C on C.name = B.name and C.status>1left join D on D.id = C.id and D.status=1(2).注意ON 子句和 WHERE 子句的不同
mysql> SELECT * FROM product LEFT JOIN product_details ON (product.id = product_details.id) AND product_details.id=2;+----+--------+------+--------+-------+| id | amount | id | weight | exist |+----+--------+------+--------+-------+| 1 | 100 | NULL | NULL | NULL || 2 | 200 | 2 | 22 | 0 || 3 | 300 | NULL | NULL | NULL || 4 | 400 | NULL | NULL | NULL |+----+--------+------+--------+-------+4 rows in set (0.00 sec) mysql> SELECT * FROM product LEFT JOIN product_details ON (product.id = product_details.id) WHERE product_details.id=2;+----+--------+----+--------+-------+| id | amount | id | weight | exist |+----+--------+----+--------+-------+| 2 | 200 | 2 | 22 | 0 |+----+--------+----+--------+-------+1 row in set (0.01 sec)从上可知,第一条查询使用 ON 条件决定了从 LEFT JOIN的 product_details表中检索符合的所有数据行。第二条查询做了简单的LEFT JOIN,然后使用 WHERE 子句从 LEFT JOIN的数据中过滤掉不符合条件的数据行。
(3).尽量避免子查询,而用join
往往性能这玩意儿,更多时候体现在数据量比较大的时候,此时,我们应该避免复杂的子查询。如下:
insert into t1(a1) select b1 from t2 where not exists(select 1 from t1 where t1.id = t2.r_id);下面这个更好
insert into t1(a1) select b1 from t2 left join (select distinct t1.id from t1 ) t1 on t1.id = t2.r_id where t1.id is null;
- Mysql----Join用法(Inner join,Left join,Right join, Cross join, Union模拟Full join)及---性能优化
- left join,inner join,right join,cross join,full join
- mysql left join,right join ,cross join inner join 用法
- mysql多表连接查询inner join, left join , right join ,full join ,cross join
- SQL join,left join ,right join , inner join , outer join用法解析及HIVE join 优化
- left join/ rigth join/ full join/ inner join/ cross join
- SQL中的各种JOIN(inner join,full outer join,left join,right join,cross join )
- SQL join、inner join、left join、right join、full join、cross join
- cross join & natural join & inner join & left outer join & right outer join & full outer join
- 【SQL】inner join、outer join、cross join和left join、right join、full join的区别
- Join, Left Join, Right Join, Cross Join, Inner Join
- join, inner join, left join, right join, full join
- oracle join、inner join、left join、right join、full join
- Join,Inner Join,Left Join,Right Join,Full Join区别
- Join,Inner Join,Left Join,Right Join,Full Join区别
- JOIN用法 INNER JOIN/LEFT JOIN/RIGHT JOIN/OUTER JOIN
- left join,inner join,right join,cross join,full join的区别[转载]
- left join,inner join,right join,cross join,full join的区别[转载]
- 机房合作之外观模式浅谈
- thrift介绍及应用(二)—简单应用
- Android AIDL的使用
- 月溅星河长路漫漫,风烟残尽独影阑珊——又是一年
- POJ 2377 最大生成树
- Mysql----Join用法(Inner join,Left join,Right join, Cross join, Union模拟Full join)及---性能优化
- Codeforces 429B B. Working out (DP)
- java类的加载 初始化时机,已经静态代码块的执行时机
- CSS 回顾
- java Servlet 接收POST数据格式
- VB 获取 Internet Explorer_Server 里面的内容
- html, xhtml, dhtml, shtml, jhtml简介
- CocoaPods
- Thrift介绍与应用(三)—hbase的thrift接口