Question

I’m make a little game in php with mysql. Now I have a problem with one of the sql query’s I created. The idea is that the query checks if the user has enough materials.

I have a query that if I use it like this it works:

SELECT
(
  SELECT COUNT(*)
  FROM building_requirements
  WHERE building_id = '1'
) as building_requirements_count,
(
  SELECT COUNT(*)
  FROM user_materials, building_requirements
  WHERE user_materials.material_id = building_requirements.material_id
    AND user_id = '27'
    AND building_id = '1'
    AND (user_material_amount >= building_material_amount) = 1
) as user_materials_count;

But when I add one column that use the result of those subquery’s it fails:

SELECT
(
  SELECT COUNT(*)
  FROM building_requirements
  WHERE building_id = '1'
) as building_requirements_count,
(
  SELECT COUNT(*)
  FROM user_materials, building_requirements
  WHERE user_materials.material_id = building_requirements.material_id
    AND user_id = '27'
    AND building_id = '1'
    AND (user_material_amount >= building_material_amount) = 1
) as user_materials_count, 
building_requirements_count = user_materials_count as enough_materials;

I get the error:

#1054 - Unknown column 'building_requirements_count' in 'field list'

Can someone explain to me why I can’t use the results of the subquery here? And how I can fix this?

Answer 1

Cause the is no field called “building_requirements_count” in your table definition.
You are not allowed to use self-defined fields here except for the WHERE part.

Why don’t you use your self-defined fields in the WHERE section of your query?

EDIT:
It would be easier for you to get each value seperate out of the DB and do the calculating stuff in PHP.

$result = mysql_query("SELECT COUNT(*) FROM building_requirements WHERE building_id = '1'");
if ($result) {
  $row = mysql_fetch_row($result);
  $building_requirements_count  = $row[0];
}
else {
  $building_requirements_count = 0;
}

$query = "  SELECT COUNT(*)
  FROM user_materials, building_requirements
  WHERE user_materials.material_id = building_requirements.material_id
    AND user_id = '27'
    AND building_id = '1'
    AND (user_material_amount >= building_material_amount) = 1";

$result2 = mysql_query($query);
if ($result2) {

  $row = mysql_fetch_row($result2);
  $user_material_count  = $row[0];
}
else {
  $user_material_count = 0;
}

$enough_materials = ( $user_material_count >= $building_requirements_count) ? true : false;