它是一个有效的查询是的，它的语法是正确的，但是按照操作顺序，where子句在select之前应用，所以@diff在执行where子句时可能为空，您可以在where子句中应用计算来克服这个问题。

SELECT dr.id, DATEDIFF(NOW(), DATE_ADD(dr.dt, INTERVAL dr.payment_session DAY)) diff
FROM `dst_request` as dr
WHERE DATEDIFF(NOW(), DATE_ADD(dr.dt, INTERVAL dr.payment_session DAY)) >= 0 
OR DATEDIFF(NOW(), DATE_ADD(dr.dt, INTERVAL dr.payment_session DAY)) <=5

https://www.eversql.com/sql-order-of-operations-sql-query-order-of-execution/

使用派生表：

SELECT 
    id, 
    diff
FROM (
        SELECT
            dr.id,
            DATEDIFF(NOW(), DATE_ADD(dr.dt, INTERVAL dr.payment_session DAY)) as diff
         FROM `dst_request` as dr
) as t
WHERE diff >= 0 OR diff <=5

原谅我，如果有语法或格式错误，因为我在浴缸里用我的手机转录这个。不过，这个概念是正确的。