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.
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 | +-----+-----------+-----+-------------+
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 | +-----+-----------+------+-------------+------------------------------------------------------------------------------------------
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 | +------+-----------+-----+-------------+
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 | +------+-----------+------+------------+
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)
#再执行:mysql> select * from A inner join B; 试一试 (与上面的结果一样) #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。
... FROM table1 INNER JOIN table2 ... FROM table1 CROSS JOIN table2 ... FROM table1 JOIN table2
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 | +------+-----------+------+-------------+
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。
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。
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;