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);
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