Php 检查其他表中是否存在id

我想检查购物车表中是否存在

product\u id

，以及是否存在定义变量

$class='show'

这是我的代码：

$article = $db->query("
    SELECT 
        b.price as price,
        b.shop_id as shop_id,
        c.image as image,
        c.name as name,
        a.id as id
    FROM products AS a
    INNER JOIN prices AS b ON a.ean = b.sku
    INNER JOIN shops as c on b.shop_id = c.id
    WHERE a.id = '$product_id'
");     
while ($data = $article->fetch_object()) {
    $price = $data->price;
    $price2 = $data->price;
    $price = number_format($price, 2, ',', ''); 
    $shop_id = $data->shop_id;
    $logo = $data->image;
    $name = $data->name;

//some html code here

}

---

现在我不知道如何检查购物车表中是否存在该产品。在这两个表中，uniqe是

产品id

检查是否返回了任何行，如果有，则设置变量

if($article->num_rows) {
   $class = "show";
} 

或者，检查属性是否不为null

if( is_null($data->name) == FALSE ) {
   $class = "show";
}

---

为此，我会使用

左连接

（请注意

案例WHEN..THEN..

部分）。它与购物车表联接，并选择一个名为

exists\u in\u cart

的列，如果购物车中存在产品，则该列的值为1，否则为0）：

如果购物车中存在产品，则您将获得1，否则为0：

$article = $db->query("
    SELECT 
        b.price as price,
        b.shop_id as shop_id,
        c.image as image,
        c.name as name,
        a.id as id,
        CASE WHEN cart.product_id IS NULL THEN 0 ELSE 1 END as exists_in_cart
    FROM products AS a
    INNER JOIN prices AS b ON a.ean = b.sku
    INNER JOIN shops as c on b.shop_id = c.id
    LEFT JOIN cart on a.id = cart.product_id
    WHERE a.id = '$product_id'
");

$productInCart = $data->exists_in_cart == 1 ? TRUE : FALSE;
if ($productInCart) {
   $class = 'show';
}
else {
   // ...
}

---

如果表中不存在产品，此查询将返回num_rows=0，说明您只选择了现有产品。

请详细解释您的问题……仅供参考：我的代码中是否缺少属性而不是函数？我需要签入名为cart的表中的位置是否存在产品，上面的代码只是第一部分，第二个不见了，那一个我不知道怎么做

SELECT 
    b.price as price,
    b.shop_id as shop_id,
    c.image as image,
    c.name as name,
    a.id as id,
    CASE WHEN cart.product_id IS NULL THEN 0 ELSE 1 END as exists_in_cart
FROM products AS a
INNER JOIN prices AS b ON a.ean = b.sku
INNER JOIN shops as c on b.shop_id = c.id
LEFT JOIN cart on a.id = cart.product_id
WHERE a.id = '$product_id'