where与having区别
<pre><code>CREATE TABLE `students` (
`stu_no` int(11) NOT NULL,
`grade` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`stu_no`,`from_date`));</code></pre>
<p>分析下面两句SQL:</p>
<pre><code>select stu_no,count(stu_no) as t from students group by stu_no HAVING t > 15；
select stu_no,count(stu_no) as t from students where count(stu_no) > 15 group by stu_no；</code></pre>
<p>1.WHERE语句在GROUP BY语句之前；SQL会在分组之前计算WHERE语句
2.HAVING语句在GROUP BY语句之后；SQL会在分组之后计算HAVING语句。所以第二种方法是错误的.
3.sql语句的执行过程是：from-->where-->group by -->having --- >order by --> select;
4.聚合函数是针对结果集进行的，where之后并不是结果集，所以主函数放在where语句中，会出现错误，
5.而having是针对结果集做筛选，用having来代替where，having一般跟在group by后</p>