检查其他表中是否存在id

Question

i want to check if product_id exist in cart table and if exist define variable $class = 'show'

this is code i have:

$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

}

now i dont know how to check if that product exist in cart table. in both tables uniqe is product_id

Answer 1

I'd use a LEFT JOIN for that purpose (notice the CASE WHEN .. THEN .. section). It joins with cart table and selects a column called exists_in_cart with a value of 1 if product exists in cart or 0 otherwise):

$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'
");

You'll then have a 1 if a product exists in a cart or 0 otherwise:

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

Answer 2

Check any rows were returned, and if there are, set your variable.

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

Or, check that a property is not null.

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

Answer 3

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'

if product doesn't exists in table this query will return num_rows = 0, you have already choosed only existing products.