如何求和不同表中的两列

Question

$username = mysqli_real_escape_string($con, $_SESSION['username']); 
$qry="(SELECT SUM(amount) from deposits) + (SELECT SUM(amount) from referral) as value_sum WHERE username ='$username' ";

我曾尝试使用它，但我不断收到此错误。

您的 SQL 语法有错误； 检查与您的 MariaDB 服务器版本相对应的手册以获取正确的语法，以在第 1 行使用近 '+ (SELECT SUM(amount) fromreferral) as value_sum WHERE username ='test0''

Answer 1

您创建两个查询并使用 UNION 将结果彼此相加：

SELECT SUM(value_sum)
  FROM ( 
         SELECT SUM(amount) as value_sum
           FROM deposits AS D
          WHERE username = '$username'
         UNION
         SELECT SUM(amount) as value_sum
           FROM referral AS R
          WHERE username = '$username'
       ) AS U