为什么我的容器 while 循环只循环第一项？

Question

// current session id
$sid = session_id();

// get current cart session
$sql = "SELECT * FROM tbl_cart WHERE ct_session_id =  '$sid'";
$result = dbQuery($sql);

// get all the items in the car category
$query = "SELECT pd_id FROM tbl_product WHERE cat_id ='28'";
$r = dbQuery($query);

//Cycle through the cart and compare each cart item to the cars to determine
  if the cart contains a car in it.
         while($cart = dbFetchAssoc($result)){
    while($product = dbFetchAssoc($r)){
        echo $cart['pd_id'] . " - ";
        echo $product['pd_id']. "<br>";
    }
}

dbFetchAssoc() 是一个自定义数据库层，基本上是 (mysql_fetch_assoc)。

我试图从查询中获取行并使用该信息进行比较。上面带有 echo 语句的代码只是为了调试目的而回显。 while 循环在嵌套循环之后退出是否有特殊原因？

Answer 1

是的。您需要再次运行查询，因为每次调用 dbFetchAssoc($r) 时，您都在推进该光标。

$sql = "SELECT * FROM tbl_cart WHERE ct_session_id =  '$sid'";
$result = dbQuery($sql);

// get all the items in the car category
$query = "SELECT pd_id FROM tbl_product WHERE cat_id ='28'";

while($cart = dbFetchAssoc($result)){
    $r = dbQuery($query);
    while($product = dbFetchAssoc($r)){
        echo $cart['pd_id'] . " - ";
        echo $product['pd_id']. "<br>";
    }
}

这是一个优化版本，它不会对数据库造成太多影响。但是，它是针对这个特定问题的，如果查询集特别大，这将是一个同样糟糕的选择——它会遇到内存问题而不是速度问题。

$sql = "SELECT * FROM tbl_cart WHERE ct_session_id =  '$sid'";
$result = dbQuery($sql);

// get all the items in the car category
$query = "SELECT pd_id FROM tbl_product WHERE cat_id ='28'";
$r = dbQuery($query);

// cache the product results into an array
$products = Array();
while($product = dbFetchAssoc($r)){
    $products[] = $product['pd_id']
}

while($cart = dbFetchAssoc($result)){
    $index = 0;
    while($product = dbFetchAssoc($r)){
        echo $cart['pd_id'] . " - ";
        echo $product[$index]. "<br>";
        $index++;
    }
}

第二个代码我没有测试过，但是思路应该够清楚了。