left join 和 left outer join 的区别
通俗的讲：http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif 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
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif table A:
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
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 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 --
<div style="">http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif举个例子：
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif 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 ----------------------------
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif 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
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif --------------------------------
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif
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
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif 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
页:
[1]