如何對SQL查詢中的兩個字段求和並為該SUM創造條件

Question

我有一個桌子students和一個桌子room

room表包含一個capacityField （int）和margeField （int）可以支持多少學生。

例如：

capacityField : 100
margeField : 12

我想檢索capacityField + margeField大於或等於學生總數的rooms 。

我做了這個查詢：

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

您不能將同一查詢的SELECT別名放在其WHERE子句中。 您要么必須使用HAVING ：

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

否則您必須重復計算：

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

順便說一句，您可以將兩個查詢合並為一個：

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，您應該學習使用准備好的查詢和bindParam而不是將變量替換為SQL字符串。