(转载) Mysql----Join用法(Inner join,Left join,Right join, Cross join, Union模拟Full join)及---性能优化

目录

• 一、Inner join
• 二、Left join
• 三、Right join
• 四、差集
• 五.Cross join
• 六.union实现Full join
• 七.性能优化

http://blog.csdn.net/ochangwen/article/details/52346610

 

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 join

 

mysql> 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 * from
table a inner join table b
on a.id = b.id;

VS

select a.*, b.*
from table a, table b
where 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 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;

下面这种写法更省时

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

(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;