爲什麼我的容器while循環僅循環第一個項目？

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（）是一個基本上是（mysql_fetch_assoc）的自定義數據庫層。

我試圖從查詢中獲取行並使用該信息進行比較。使用echo語句的上面的代碼只是迴應調試目的。是否有一個特定的原因，循環嵌套後退出while循環？

Answer 1

是的。您需要再次運行查詢，因爲每次您致電dbFetchAssoc($r)時，都會推進該光標。

$sql = "SELECT * FROM tbl_cart WHERE ct_session_id = '$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>"; 
    } 
} 

這是一個優化的版本，它並沒有像數據庫那麼多。但是，對於這個特定的問題，這是特定的，如果查詢集特別大，那麼它也是一個同樣不好的選擇 - 它會遇到內存問題而不是速度問題。

$sql = "SELECT * FROM tbl_cart WHERE ct_session_id = '$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++; 
    } 
} 

我還沒有測試過這個第二個代碼，但這個想法應該足夠清楚。