实际上不需要同时查询这两个表。由于客户id在stocks表中可用，因此您只需要：

SELECT
    s.customer_id,
    s.id,
    s.item,
    s.color
FROM tbl_stocks as s
WHERE s.customer_id = ?

如果要查询这两个表，例如检索其他一些客户信息，请使用联接：

SELECT
    c.customer_id,
    s.id,
    s.item,
    s.color
FROM
   tbl_customers as c
   INNER JOIN  tbl_stocks as s on s.customer_id = c.customer_id
WHERE c.customer_id = ?

你应该使用 joins 这是您可以尝试的查询。

select c.*, s.*
from tbl_customers c
join tbl_stocks s on s.customer_id = c.customer_id
and c.customer_id = 1

此外，你应该阅读更多关于https://dev.mysql.com/doc/refman/8.0/en/join.html
在这里，我将更新php页面中的answer w.r.t打印值

$dbConn = mysqli_connect('localhost', 'username', 'password', 'databaseName');
if(!$dbConn){
    echo "DB Not connected !";
}

$query = "select c.*, s.* from tbl_customers c join tbl_stocks s on s.customer_id = c.customer_id and c.customer_id = 1";
$result= $dbConn->query($query);
while($row = $result->fetch_object()){ 
    echo $row->customer_id;
    echo $row->color;
    // here you may add more columns to print
}

了解sql联接：

SELECT tbl_customers.customer_id,tbl_stocks.* 
FROM tbl_customers 
LEFT JOIN tbl_stocks 
    ON (tbl_customers.customer_id=tbl_stocks.customer_id)