MySQL强制查询返回NULL [英] MySQL force query to return NULL

问题描述

我有以下几点:

SELECT q25, COUNT( q25 ) AS count, AVG (q1) AS q1
FROM tresults WHERE id = 'Yes' AND date = 'MARCH2010' AND q25 = '1' 
GROUP BY q25

目前，查询返回 MySQL 返回一个空结果集(即零行). 这是正确的 - 是否有可能让它返回 NULL 代替?

At the moment, the query returns MySQL returned an empty result set (i.e. zero rows). which is correct - is it possible to get it to return NULL instead?

或

PHP中有没有办法在事件之后处理这个，比如:

Is there a way of dealing with this after the event in PHP, such as:

$resultprev = mysql_query($queryprev);
if($resultprev == ''){
// do something
}

推荐答案

use mysql_num_rows :

$resultprev = mysql_query($queryprev);
$num_rows = mysql_num_rows($resultprev);
if($num_rows == 0){
   // 0 results !
}

或者你可以联合一个空行:

or you could union a null row :

SELECT q25, COUNT( q25 ) AS count, AVG (q1) AS q1
FROM tresults WHERE id = 'Yes' AND date = 'MARCH2010' AND q25 = '1' 
GROUP BY q25

UNION

SELECT null, null as count, null as q1

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