Oracle 11g SQL学习笔记1——Using Aggregate F..

1.聚集函数（分组函数）有哪些？

AVG(x)                                
COUNT(x)                               //记录行数
MAX(x)
MEDIAN(x)                              //取中值    
MIN(x)
STDDEV(x)                              //求标准差
SUM(x)
VARIANCE(x)                            //求方差

■ The Oracle database has two main groups of functions: single-row functions and
aggregate functions.
■ Single-row functions operate on one row at a time and return one row of output for
each input row. There are five main types of single-row functions: character functions,
numeric functions, conversion functions, date functions, and regular expression
functions.
■ Aggregate functions operate on multiple rows and return one row of output.
■ Blocks of rows may be grouped together using the GROUP BY clause.
■ Groups of rows may be filtered using the HAVING clause.

2.SELECT语句的执行顺序（含聚集函数即分组函数）

SQL中使用分组函数（GROUP FUNCTIONS）时，我们常常引入HAVING子句来进一步限制条件（受WHERE子句位置限制）,这样SELECT语句就变长了，通常是这样一个格 式：

SELECT         column,group_function          //group函数之外的列必须处在group by子句中，也就是说column必须二选一

[WHERE        condition]                          //where子句中不能包含group函数，因为它先于group函数执行

[ORDER BY     column]                       //只有order by下可以使用别名