left join 和 left outer join 的区别
2007-09-07 14:51
232 查看
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通俗的讲:
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A left join B 的连接的记录数与A表的记录数同
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
A right join B 的连接的记录数与B表的记录数同
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
A left join B 等价B right join A
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![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
table A:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
Field_K, Field_A
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 b
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
4 c
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
table B:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
Field_K, Field_B
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 x
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 y
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
4 z
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select a.Field_K, a.Field_A, b.Field_K, b.Field_B
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from a left join b on a.Field_K=b.Field_K
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
Field_K Field_A Field_K Field_B
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---------- ---------- ---------- ----------
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1 a 1 x
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 b NULL NULL
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
4 c 4 z
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select a.Field_K, a.Field_A, b.Field_K, b.Field_B
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from a right join b on a.Field_K=b.Field_K
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
Field_K Field_A Field_K Field_B
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
---------- ---------- ---------- ----------
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a 1 x
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
NULL NULL 2 y
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
4 c 4 z
--
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举个例子:
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假设a表和b表的数据是这样的。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
a b
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
id name id stock
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a 1 15
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 b 2 50
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 c
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select * from a inner join b on a.id=b.id
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
这个语法是连接查询中的内连接,它产生的结果是
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
两个表相匹配的记录出现在结果列表中。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
根据上面的表,出现的结果是这样的
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
a.id name b.id stock
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a 1 15
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 b 2 50
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
----------------------------
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select * from a,b where a.id=b.id
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
这个语法是内连接的另外一种写法,其执行结果与inner join 一样
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![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
--------------------------------
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![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select * from a left/right join b on a.id=b.id
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
这个是外连接语法中的左外连接或右外连接
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如果是左外连接的话,它将显示a表的所有记录,
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select a.*,b.* from a left join b on a.id=b.id
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
查询的结果是这样的:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
a.id name b.id stock
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a 1 15
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 b 2 50
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 c null null
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
--------------------------------------------
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
如果是右外连接的话,它将显示b表的所有记录,
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select a.*,b.* from a right join b on a.id=b.id
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
查询的结果是这样的:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
a.id name b.id stock
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a 1 15
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 b 2 50
--
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select a.*,b.* from a left join b on a.k = b.k
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select a.*,b.* from a left outer join b on a.k =b.k
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
----------上面两种一样left join是left outer join的简写
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select a.*,b.* from a left inner join b on a.k = b.k
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
没有这种写法,错误的语句.
--
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在你要使用多个left join的时候
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比如说10个
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
我们把10个全都写成left join的形式
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
然后再SQL让他自动运行一下,它会把最后一次出现的left join变成left outer join
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
所以依此推理,最后一个left join会以left outer join的形式存在
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
当然,不管变不变对结果的显示没有任何影响
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
希望我的实验能对你有所帮助
--
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使用关系代数合并数据
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1 关系代数
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合并数据集合的理论基础是关系代数,它是由E.F.Codd于1970年提出的。
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在关系代数的形式化语言中:
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用表、或者数据集合表示关系或者实体。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
用行表示元组。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
用列表示属性。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
关系代数包含以下8个关系运算符
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选取――返回满足指定条件的行。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
投影――从数据集合中返回指定的列。
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笛卡尔积――是关系的乘法,它将分别来自两个数据集合中的行以所有可能的方式进行组合。
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并――关系的加法和减法,它可以在行的方向上合并两个表中的数据,就像把一个表垒在另一个表之上一样。
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交――返回两个数据集合所共有的行。
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差――返回只属于一个数据集合的行。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
连接――在水平方向上合并两个表,其方法是:将两个表中在共同数据项上相互匹配的那些行合并起来。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
除――返回两个数据集之间的精确匹配。
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此外,作为一种实现现代关系代数运算的方法,SQL还提供了:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
子查询――类似于连接,但更灵活;在外部查询中,方式可以使用表达式、列表或者数据集合的地方都可以使用子查询的结果。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
本章将主要讲述多种类型的连接、简单的和相关的子查询、几种类型的并、关系除以及其他的内容。
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2 使用连接
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2.1 连接类型
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在关系代数中,连接运算是由一个笛卡尔积运算和一个选取运算构成的。首先用笛卡尔积完成对两个数据集合的乘运算,然后对生成的结果集合进行选取运算,确保只把分别来自两个数据集合并且具有重叠部分的行合并在一起。连接的全部意义在于在水平方向上合并两个数据集合(通常是表),并产生一个新的结果集合,其方法是将一个数据源中的行于另一个数据源中和它匹配的行组合成一个新元组。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
SQL提供了多种类型的连接方式,它们之间的区别在于:从相互交叠的不同数据集合中选择用于连接的行时所采用的方法不同。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
连接类型 定义
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内连接 只连接匹配的行
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
左外连接 包含左边表的全部行(不管右边的表中是否存在与它们匹配的行),以及右边表中全部匹配的行
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
右外连接 包含右边表的全部行(不管左边的表中是否存在与它们匹配的行),以及左边表中全部匹配的行
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
全外连接 包含左、右两个表的全部行,不管另外一边的表中是否存在与它们匹配的行。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
(H)(theta)连接 使用等值以外的条件来匹配左、右两个表中的行
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
交叉连接 生成笛卡尔积-它不使用任何匹配或者选取条件,而是直接将一个数据源中的每个行与另一个数据源的每个行都一一匹配
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
在INFORMIX中连接表的查询
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如果FROM子句指定了多于一个表引用,则查询会连接来自多个表的行。连接条件指定各列之间(每个表至少一列)进行连接的关系。因为正在比较连接条件中的列,所以它们必须具有一致的数据类型。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
SELECT语句的FROM子句可以指定以下几种类型的连接
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
FROM子句关键字 相应的结果集
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
CROSS JOIN 笛卡尔乘积(所有可能的行对)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
INNER JOIN 仅对满足连接条件的CROSS中的列
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
LEFT OUTER JOIN 一个表满足条件的行,和另一个表的所有行
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
RIGHT OUTER JOIN 与LEFT相同,但两个表的角色互换
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
FULL OUTER JOIN LEFT OUTER 和 RIGHT OUTER中所有行的超集
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2.2 内连接(Inner Join)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
内连接是最常见的一种连接,它页被称为普通连接,而E.FCodd最早称之为自然连接。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
下面是ANSI SQL-92标准
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select *
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from t_institution i
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
inner join t_teller t
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on i.inst_no = t.inst_no
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
where i.inst_no = "5801"
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
其中inner可以省略。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
等价于早期的连接语法
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select *
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from t_institution i, t_teller t
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
where i.inst_no = t.inst_no
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
and i.inst_no = "5801"
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2.3 外连接
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2.3.1 左外连接(Left Outer Jion)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select *
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from t_institution i
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
left outer join t_teller t
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on i.inst_no = t.inst_no
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
其中outer可以省略。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2.3.2 右外连接(Rigt Outer Jion)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select *
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from t_institution i
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
right outer join t_teller t
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on i.inst_no = t.inst_no
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2.3.3 全外连接(Full Outer)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
全外连接返回参与连接的两个数据集合中的全部数据,无论它们是否具有与之相匹配的行。在功能上,它等价于对这两个数据集合分别进行左外连接和右外连接,然后再使用消去重复行的并操作将上述两个结果集合并为一个结果集。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
在现实生活中,参照完整性约束可以减少对于全外连接的使用,一般情况下左外连接就足够了。在数据库中没有利用清晰、规范的约束来防范错误数据情况下,全外连接就变得非常有用了,你可以使用它来清理数据库中的数据。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select *
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from t_institution i
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
full outer join t_teller t
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on i.inst_no = t.inst_no
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2.3.4 外连接与条件配合使用
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
当在内连接查询中加入条件是,无论是将它加入到join子句,还是加入到where子句,其效果是完全一样的,但对于外连接情况就不同了。当把条件加入到 join子句时,SQL Server、Informix会返回外连接表的全部行,然后使用指定的条件返回第二个表的行。如果将条件放到where子句中,SQL Server将会首先进行连接操作,然后使用where子句对连接后的行进行筛选。下面的两个查询展示了条件放置位子对执行结果的影响:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
条件在join子句
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select *
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from t_institution i
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
left outer join t_teller t
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on i.inst_no = t.inst_no
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
and i.inst_no = “5801”
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果是:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
inst_no inst_name inst_no teller_no teller_name
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
5801 天河区 5801 0001 tom
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
5801 天河区 5801 0002 david
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
5802 越秀区
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
5803 白云区
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
条件在where子句
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select *
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from t_institution i
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
left outer join t_teller t
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on i.inst_no = t.inst_no
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
where i.inst_no = “5801”
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果是:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
inst_no inst_name inst_no teller_no teller_name
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
5801 天河区 5801 0001 tom
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
5801 天河区 5801 0002 david
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2.4 自身连接
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
自身连接是指同一个表自己与自己进行连接。这种一元连接通常用于从自反关系(也称作递归关系)中抽取数据。例如人力资源数据库中雇员与老板的关系。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
下面例子是在机构表中查找本机构和上级机构的信息。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select s.inst_no superior_inst, s.inst_name sup_inst_name, i.inst_no, i.inst_name
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from t_institution i
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
join t_institution s
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on i.superior_inst = s.inst_no
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果是:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
superior_inst sup_inst_name inst_no inst_name
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
800 广州市 5801 天河区
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
800 广州市 5802 越秀区
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
800 广州市 5803 白云区
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2.5 交叉(无限制) 连接
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
交叉连接用于对两个源表进行纯关系代数的乘运算。它不使用连接条件来限制结果集合,而是将分别来自两个数据源中的行以所有可能的方式进行组合。数据集合中一的每个行都要与数据集合二中的每一个行分别组成一个新的行。例如,如果第一个数据源中有5个行,而第二个数据源中有4个行,那么在它们之间进行交叉连接就会产生20个行。人们将这种类型的结果集称为笛卡尔乘积。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
大多数交叉连接都是由于错误操作而造成的;但是它们却非常适合向数据库中填充例子数据,或者预先创建一些空行以便为程序执行期间所要填充的数据保留空间。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select *
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
from t_institution i
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
cross join t_teller t
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
在交叉连接中没有on条件子句
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 APPENDIX
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3.1 A 参考资料与资源
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
《Microsoft SQL Server 2000 Bile》Paul Nielsen
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
Paul Nielsen的Web站点
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
www.isnotnull.com
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3.2 注文章所有SQL在IBM Informix Dynamic Server Version 9.40.TC2E1测试通过
--
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
表A记录如下:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
aID aNum
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a20050111
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 a20050112
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 a20050113
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
4 a20050114
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
5 a20050115
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
表B记录如下:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
bID bName
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 2006032401
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 2006032402
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 2006032403
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
4 2006032404
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
8 2006032408
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
实验如下:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1.left join
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
sql语句如下:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select * from A
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
left join B
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on A.aID = B.bID
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果如下:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
aID aNum bID bName
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a20050111 1 2006032401
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 a20050112 2 2006032402
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 a20050113 3 2006032403
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
4 a20050114 4 2006032404
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
5 a20050115 NULL NULL
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
(所影响的行数为 5 行)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果说明:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
left join是以A表的记录为基础的,A可以看成左表,B可以看成右表,left join是以左表为准的.
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
换句话说,左表(A)的记录将会全部表示出来,而右表(B)只会显示符合搜索条件的记录(例子中为: A.aID = B.bID).
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
B表记录不足的地方均为NULL.
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2.right join
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
sql语句如下:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select * from A
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
right join B
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on A.aID = B.bID
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果如下:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
aID aNum bID bName
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a20050111 1 2006032401
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 a20050112 2 2006032402
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 a20050113 3 2006032403
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
4 a20050114 4 2006032404
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
NULL NULL 8 2006032408
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
(所影响的行数为 5 行)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果说明:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
仔细观察一下,就会发现,和left join的结果刚好相反,这次是以右表(B)为基础的,A表不足的地方用NULL填充.
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3.inner join
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
sql语句如下:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
select * from A
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
innerjoin B
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
on A.aID = B.bID
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果如下:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
aID aNum bID bName
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
1 a20050111 1 2006032401
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
2 a20050112 2 2006032402
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
3 a20050113 3 2006032403
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
4 a20050114 4 2006032404
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
结果说明:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
很明显,这里只显示出了 A.aID = B.bID的记录.这说明inner join并不以谁为基础,它只显示符合条件的记录.
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
-----------------[以下为网上的一点资料]------------------
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
LEFT JOIN操作用于在任何的 FROM 子句中,组合来源表的记录。使用 LEFT JOIN 运算来创建一个左边外部联接。左边外部联接将包含了从第一个(左边)开始的两个表中的全部记录,即使在第二个(右边)表中并没有相符值的记录。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
语法:FROM table1 LEFT JOIN table2 ON table1.field1 compopr table2.field2
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
说明:table1, table2参数用于指定要将记录组合的表的名称。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
field1, field2参数指定被联接的字段的名称。且这些字段必须有相同的数据类型及包含相同类型的数据,但它们不需要有相同的名称。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
compopr参数指定关系比较运算符:"=", "<", ">", "<=", ">=" 或 "<>"。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
如果在INNER JOIN操作中要联接包含Memo 数据类型或 OLE Object 数据类型数据的字段,将会发生错误。
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