SQL的聚合函数与排序

count 函数

语法：

Select count(*)|count(列名) from table_name [where where_defination];Select count(id) from stu;Select count(name) from stu where math > 90;select count (name) from stu where chines+math+English> 250;

sum函数

Select sum (列名) from stu [where where_defination];
Select sum (列名),sum(列名), sum(列名) from stu [where where_definition];
Select sum (列名)+sum(列名)+ sum(列名) from stu [where where_definition];
Select sum (列名)+sum(列名)+sum(列名) as 总分 from stu [where where_definition];
Select sum (列名)/count(name) from stu [where where_defination];
Select avg(math) from stu;
Select avg(math + chines + english) from stu;
Select max|min(列名) from stu;
Select max(math + chines + english) from stu;
Select min(math + chines + english) from stu;

对查询结果排序

使用order by 子句排序查询结果

Select column1, column2, column3 from table order by column asc|desc

order by指排序的列，也可以是select后面指定的列名

order by 子句应位于select 语句的结尾

Select name, math from stu order by math;
select name, math+english+chines from stu order by math+english+chines desc;
select name, math+english+chines from stu where name like 'zhang%' order by math+english+chines desc;

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