Question

我有以下内容：

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 returned an empty result set (i.e. zero rows).这是正确的 - 是否可以让它返回NULL？

或

有没有办法在PHP事件之后处理这个问题，例如：

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

Answer 1

使用mysql_num_rows：

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

或者你可以结合一个空行：

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

Answer 2

试试这个

if(mysql_affected_rows($res)) {
    //your stuff
}

或在sql中

SELECT (
    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, null

) AS x
LIMIT 1

MySQL强制查询返回NULL

2 个答案: