PHP - SQL select first column with no data - php

Question

How do I find the first column with no data?
//column pet1='dog'
//column pet2=''
//column pet3=''
//column pet4='cat'
if($F=="getempty"){
$uid=$_GET["uid"];
$sql = mysql_query("SELECT DISTINCT pet1,pet2,pet3,pet4 FROM a WHERE uid='".$uid."' AND pet1='' OR pet2='' OR pet3=''OR pet4=''");
while($column=mysql_fetch_field($sql)){
$empty=$column->name;
echo json_encode(array("empty"=>$empty));
}
}else{}
I keep trying but so far it just returns all column names if one is empty
pet1,pet2,pet3,pet4

Answer 1

DISTINCT will combine the rows which are exactly alike (having all the selected columns the same) and return them.
To get the name of the first empty column, use a query like:
SELECT
CASE
WHEN pet1 = '' THEN 'pet1'
WHEN pet2 = '' THEN 'pet2'
WHEN pet3 = '' THEN 'pet3'
WHEN pet4 = '' THEN 'pet4'
ELSE NULL
END AS firstempty
FROM a
WHERE uid = '$uid';
The idea is that the CASE statement checks each one in order, and finding an empty one returns the column name without checking the rest of them.
Don't forget to escape $uid with mysql_real_escape_string() before passing it to the query.
$uid = mysql_real_escape_string($uid);
Since we're now getting the column name inside a field called firstempty, we'll also need to change the way it's fetched:
while($row = mysql_fetch_assoc($sql)){
$empty = $row['firstempty'];
echo json_encode(array("empty"=>$empty));
}