MysQL：如何将条件（如果）的结果保存到变量中？

Question

有什么办法可以避免重复“如果”？

SELECT 
    a.a_id,
    IF(acd.dt_changed IS NULL, a.dt, acd.dt_changed) AS dt
FROM 
    articles a
LEFT JOIN 
    articles_changed_dt acd 
ON 
    a.a_id = acd.a_id
WHERE 
    a.status = 1 
    AND IF(acd.dt_changed IS NULL, a.dt, acd.dt_changed) 
    BETWEEN '2025-02-06 00:00:00' AND '2025-02-10 23:59:59';

Answer 1

使用

COALESCE()

：

SELECT 
    a.a_id,
    COALESCE(acd.dt_changed, a.dt) AS dt
FROM articles a
LEFT JOIN articles_changed_dt acd 
    ON a.a_id = acd.a_id
WHERE 
    a.status = 1 AND
    COALESCE(acd.dt_changed, a.dt) BETWEEN '2025-02-06 00:00:00' AND '2025-02-10 23:59:59';