如何对SQL查询中的两个字段求和并为该SUM创造条件

Question

我有一个桌子students和一个桌子room

room表包含一个capacityField （int）和margeField （int）可以支持多少学生。

例如：

capacityField : 100
margeField : 12

我想检索capacityField + margeField大于或等于学生总数的rooms 。

我做了这个查询：

public function fetchSalle(){
    $cmod = $_GET['cmod'];
    $sth = $this->db->prepare("SELECT * FROM etudiants AS etd INNER JOIN filieres AS flr WHERE etd.id_filiere=flr.id_filiere AND etd.id_filiere=$cmod");
    $sth->execute();
    $total = $sth->rowCount();      

    $stmt = $this->db->prepare("SELECT *, (capaciteField + margeField) AS Sum FROM salles WHERE Sum>=$total");      
    $stmt->execute();

    return $stmt->fetchAll();       
}

但是我不工作

Answer 1

您不能将同一查询的SELECT别名放在其WHERE子句中。 您要么必须使用HAVING ：

SELECT *, (capaciteField + margeField) AS Sum 
FROM salles 
HAVING Sum>=$total

否则您必须重复计算：

SELECT *, (capaciteField + margeField) AS Sum 
FROM salles 
WHERE (capaciteField + margeField) >= $total

顺便说一句，您可以将两个查询合并为一个：

SELECT s.*, (capaciteField + margeField) AS Sum
FROM salles AS s
JOIN (SELECT COUNT(*) AS total
      FROM etudiants AS e
      JOIN filieres AS f ON e.id_filiere = f.id_filiere
      WHERE e.id_filiere = $cmod) AS t
WHERE (capaciteField + margeField) > total

另外，您应该学习使用准备好的查询和bindParam而不是将变量替换为SQL字符串。