您只需要在where子句中添加逻辑即可：

SELECT `p_Id`,`user_id`,`doc_id`,`credit` ,`app_date`,`expires_on`,
       (credit -debited_amount) AS credit
FROM `wp_loyalty_credits`
WHERE `expires_on`>now() and (credit > debited_amount or debited_amount is null);

您的查询credit在中重新定义select。但是，这无关紧要，因为您不能在where子句中引用列别名。因此，该列credit就是它所使用的。如果添加表别名，则更加清楚：

SELECT lc.p_Id, lc.user_id, lc.doc_id, lc.credit, lc.app_date, lc.expires_on,
       (lc.credit - lc.debited_amount) AS credit
FROM `wp_loyalty_credits` lc
WHERE lc.expires_on > now() and
      (lc.credit > lc.debited_amount or lc.debited_amount is null);