php的子查詢

Question

很抱歉，如果這可能是一個重復的問題，但是我迫切需要項目提交的幫助。

我有一個數據庫停車場，其中有2個表，其中是carpark_availability和歷史記錄。

從我的代碼中，我無法從表carpark_availability中獲取字段“ development”。 我想問一下如何構造我的查詢，如果我想從h.development = c.development的c中選擇*。

如下所示是我的代碼：

<?php
session_start();
    $con = new mysqli('localhost','root','','carpark_project');
    $dev = $_SESSION["development"]; 
    echo "Development: ";
    echo $dev;



    $sql = "select * from carpark_availability where 
            carpark_availability.Development IN (select history.development 
            from history where
            history.Development = '$dev' )";
   //$sql = "select * from carpark_availability where development = '$dev'";
   //$sql = "select * from carpark_availability where (select Development
   //from history where Development= '$dev'";

   $result = $con->query($sql);


   while($row = $result ->fetch_assoc())
   { 

        echo "Development: ";
        echo $row["Development"]. "</br>";
        echo $row["Area"]."</br>";
        echo $row["weekday1"]."</br>";
    } 

 ?>

Answer 1

聯接比子查詢容易得多。

SELECT c.*
FROM carpark_availability c
INNER JOIN history
USING (development)
GROUP BY development;

我猜測您收到錯誤消息是因為您的查詢未返回任何結果，因此$result設置為boolean false 。 您的代碼中沒有任何要檢查的內容，因此您嘗試在其上調用fetch_assoc()