Mysql实例浅析Mysql Join语法以及性能优化
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《Mysql实例浅析Mysql Join语法以及性能优化》要点： 一．Join语法概述MYSQL学习 join 用于多表中字段之间的联系,语法如下：MYSQL学习 代码如下:... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON conditiona table1:左表；table2:右表.MYSQL学习 JOIN 按照功能大致分为如下三类：MYSQL学习 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.MYSQL学习 LEFT JOIN(左连接)：取得左表(table1)完全记录,即是右表(table2)并无对应匹配记录.MYSQL学习 RIGHT JOIN(右连接)：与 LEFT JOIN 相反,取得右表(table2)完全记录,即是左表(table1)并无匹配对应记录.MYSQL学习 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.MYSQL学习 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学习 代码如下:mysql> select A.id,A.name,B.name from A,B where A.id=B.id; +----+-----------+-------------+ | id | name?????? | name???????????? | +----+-----------+-------------+ |? 1 | Pirate?????? | Rutabaga????? | |? 2 | Monkey??? | Pirate??????????? | |? 3 | Ninja???????? | Darth Vader | |? 4 | Spaghetti? | Ninja???????????? | +----+-----------+-------------+ 4 rows in set (0.00 sec) 二.Inner joinMYSQL学习 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.MYSQL学习 代码如下:mysql> select * from A inner join B on A.name = B.name; +----+--------+----+--------+ | id | name?? | id | name?? | +----+--------+----+--------+ |? 1 | Pirate |? 2 | Pirate | |? 3 | Ninja? |? 4 | Ninja? | +----+--------+----+--------+
三.Left joinMYSQL学习 代码如下:mysql> select * from A left join B on A.name = B.name; #或者：select * from A left outer join B on A.name = B.name; +----+-----------+------+--------+ 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学习
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学习 代码如下:mysql> select * from A left join B on A.name=B.name where A.id is null or B.id is null; +----+-----------+------+------+ | id | name????? | id?? | name | +----+-----------+------+------+ |? 2 | Monkey??? | NULL | NULL | |? 4 | Spaghetti | NULL | NULL | +----+-----------+------+------+ 2 rows in set (0.00 sec)
同理,还可以模拟inner join. 如下：MYSQL学习 代码如下:mysql> select * from A left join B on A.name=B.name where A.id is not null and B.id is not null; +----+--------+------+--------+ | id | name?? | id?? | name?? | +----+--------+------+--------+ |? 1 | Pirate |??? 2 | Pirate | |? 3 | Ninja? |??? 4 | Ninja? | +----+--------+------+--------+ 2 rows in set (0.00 sec) 求差集：MYSQL学习 根据上面的例子可以求差集,如下：MYSQL学习 代码如下:SELECT * FROM A LEFT JOIN B ON A.name = B.name WHERE B.id IS NULL union SELECT * FROM A right JOIN B ON A.name = B.name WHERE A.id IS NULL; # 结果 ??? +------+-----------+------+-------------+ | id?? | name????? | id?? | name??????? | +------+-----------+------+-------------+ |??? 2 | Monkey??? | NULL | NULL??????? | |??? 4 | Spaghetti | NULL | NULL??????? | | NULL | NULL????? |??? 1 | Rutabaga??? | | NULL | NULL????? |??? 3 | Darth Vader | +------+-----------+------+-------------+ MYSQL学习 四.Right joinMYSQL学习 代码如下:mysql> select * from A right join B on A.name = B.name; +------+--------+----+-------------+ | id?? | name?? | id | name??????? | +------+--------+----+-------------+ | NULL | NULL?? |? 1 | Rutabaga??? | |??? 1 | Pirate |? 2 | Pirate????? | | NULL | NULL?? |? 3 | Darth Vader | |??? 3 | Ninja? |? 4 | Ninja?????? | +------+--------+----+-------------+ 4 rows in set (0.00 sec) 同left join.MYSQL学习 五.Cross joinMYSQL学习 cross join：交叉连接,得到的结果是两个表的乘积,即笛卡尔积MYSQL学习 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),2),(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学习 代码如下:mysql> select * from A cross join B; +----+-----------+----+-------------+ | id | name????? | id | name??????? | +----+-----------+----+-------------+ |? 1 | Pirate??? |? 1 | Rutabaga??? | |? 2 | Monkey??? |? 1 | Rutabaga??? | |? 3 | Ninja???? |? 1 | Rutabaga??? | |? 4 | Spaghetti |? 1 | Rutabaga??? | |? 1 | Pirate??? |? 2 | Pirate????? | |? 2 | Monkey??? |? 2 | Pirate????? | |? 3 | Ninja???? |? 2 | Pirate????? | |? 4 | Spaghetti |? 2 | Pirate????? | |? 1 | Pirate??? |? 3 | Darth Vader | |? 2 | Monkey??? |? 3 | Darth Vader | |? 3 | Ninja???? |? 3 | Darth Vader | |? 4 | Spaghetti |? 3 | Darth Vader | |? 1 | Pirate??? |? 4 | Ninja?????? | |? 2 | Monkey??? |? 4 | Ninja?????? | |? 3 | Ninja???? |? 4 | Ninja?????? | |? 4 | Spaghetti |? 4 | Ninja?????? | +----+-----------+----+-------------+ 16 rows in set (0.00 sec) #再执行：mysql> select * from A inner join B; 试一试MYSQL学习 #在执行mysql> select * from A cross join B on A.name = B.name; 试一试 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：MYSQL学习 代码如下:... FROM table1 INNER JOIN table2 ... FROM table1 CROSS JOIN table2 ... FROM table1 JOIN table2 六.Full joinMYSQL学习 代码如下:mysql> select * from A left join B on B.name = A.name ??? -> union ??? -> select * from A right join B on B.name = A.name; +------+-----------+------+-------------+ | id?? | name????? | id?? | name??????? | +------+-----------+------+-------------+ |??? 1 | Pirate??? |??? 2 | Pirate????? | |??? 2 | Monkey??? | NULL | NULL??????? | |??? 3 | Ninja???? |??? 4 | Ninja?????? | |??? 4 | Spaghetti | NULL | NULL??????? | | NULL | NULL????? |??? 1 | Rutabaga??? | | NULL | NULL????? |??? 3 | Darth Vader | +------+-----------+------+-------------+ 6 rows in set (0.00 sec) 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.MYSQL学习
七.性能优化 如：MYSQL学习 代码如下:select * from table a inner join table b on a.id = b.id; VSMYSQL学习 代码如下:select a.*,b.* from table a,table b where a.id = b.id; 我在数据库中比较(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.MYSQL学习 2.left join/right join VS inner joinMYSQL学习 尽量用inner join.避免 LEFT JOIN 和 NULL.MYSQL学习 在使用left join(或right join)时,应该清楚的知道以下几点： ??? 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.MYSQL学习 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：MYSQL学习 PASSMYSQL学习 代码如下:select * from A inner join B on B.name = A.name left join C on C.name = B.name left join D on D.id = C.id where C.status>1 and D.status=1; GreatMYSQL学习 代码如下:select * from A inner join B on B.name = A.name left join C on C.name = B.name and C.status>1 left join D on D.id = C.id and D.status=1 从上面例子可以看出,尽可能满足ON的条件,而少用Where的条件.从执行性能来看第二个显然更加省时.MYSQL学习 (2).注意ON 子句和 WHERE 子句的不同MYSQL学习 如作者举了一个列子：MYSQL学习 代码如下: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 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.MYSQL学习 (3).尽量避免子查询,而用joinMYSQL学习 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：MYSQL学习 PASSMYSQL学习 代码如下:insert into t1(a1) select b1 from t2 where not exists(select 1 from t1 where t1.id = t2.r_id); GreatMYSQL学习 代码如下: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;? （编辑：李大同） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
