选择MAX（ad_number）问题的查询

Question

$r = ca_mysql_query("SELECT MAX(ad_number) FROM `myTable` WHERE  `user` = {$user_id} ");

$max = $r[0]['ad_number'];
echo $max;

它应该打印max ad_number但它没有返回任何值
wheares， 如果我尝试查询从数组中删除MAX（ad_number）其returnig字段数据。

$r = ca_mysql_query("SELECT * FROM `myTable` WHERE  `user` = {$user_id} ");

$max = $r[0]['ad_number'];

返回给定记录的值

什么是错的？

Answer 1

$r = ca_mysql_query("SELECT MAX(ad_number) AS AD_NUMBER FROM myTable WHERE user = {$user_id} ");

$max = $r[0]['AD_NUMBER']; 
echo $max;

Answer 2

为您使用别名列：

SELECT MAX(ad_number) as max_ad_number FROM myTable WHERE  user = {$user_id}

然后这样做：

$max = $r[0]['max_ad_number']; echo $max;

请参阅w3schools here

中的文档