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SQL 集合（笔记）

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Jun 07, 2016 pm 04:33 PM

ororacledatabase

Oracle是关系型数据，其中的数据表都是有一定规律的数据的一个个集合，所以在使用SQL时，如果能按照集合的思路来进行时会节省很多

SQL是关于集合的

Oracle是关系型数据，其中的数据表都是有一定规律的数据的一个个集合，所以在使用SQL时，如果能按照集合的思路来进行时会节省很多效率，也鞥让语句更加的清晰明了。

1、四个集合运算符

1）union 返回不重复的数据行；

2）union all 返回所有的数据行（包括重复的）；

3）minus 返回第一个输入查询存在，，其他查询不存在的数据行；

4）intersect 返回所有数据查询中都存在的数据行。

注：除union all 以外的集合运算都需要对结果进行排序/取唯一值操作，这就意味着需要更多地效率，所以在不关心重复的情况下尽量使用union all

2、关于空值（null）

关系模型只有真、假两个结果；而SQL语言允许有三个结果：真、假、未知。

而对于未知这种结果，oracle会认为是正确的，于是在很多的搜索条件中都必须要注意这一点，否则搜索出来的结果和预期会有很大的出入

order by 语句默认把null值放在最后输出，如果要把它放在前边，需要在order by 语句后面加上 nulls first

null在聚合函数中，会被自动的去除掉，也就是不参与聚合函数的运算

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