1.左关联一个新的子表

-- 全选出库申请单:
SELECT
        ap.QUARTER_ID , ap.OUT_APPLY_ID , ap.APPLY_TYPE , ap.REGION_ID , ap.REGION_NAME , ap.RES_TYPE ,
        ac.ACTIVITY_BEGIN_DATE , ac.ACTIVITY_END_DATE , ac.APPLY_TIME , ap.FILL_USER_NAME , ap.STATUS , 
        FROM out_apply ap
        LEFT JOIN activity ac
        ON ap.ACTIVITY_ID = ac.ACTIVITY_ID
        LEFT JOIN (select TIMESTAMPDIFF(DAY,ACTIVITY_BEGIN_DATE,NOW()) AS mdate , acc.ACTIVITY_ID FROM activity acc) ab 
        on ac.ACTIVITY_ID = ab.ACTIVITY_ID
        WHERE 1 = 1
        and ap.REGION_ID = '1'
        AND ap.RES_TYPE = '1'
        AND ab.mdate  = '331'
        AND ap.APPLY_TYPE = '1'
        AND ap.QUARTER_ID = '1'
        AND ap.OUT_APPLY_NO = '1'
ORDER BY ap.ACTIVITY_ID;

子查询:
select TIMESTAMPDIFF(DAY,ACTIVITY_BEGIN_DATE,NOW()) FROM activity;

2.直接在里面查询
写成TIMESTAMPDIFF(DAY,NOW(),ac.ACTIVITY_BEGIN_DATE) AS CLOSE_DATE ,函数的形式
但是在AND条件里不能用 CLOSE_DATE 这个属性 , 因为查询的过程中函数还在运行 , 可能查不到 , 所以AND条件里还需要再去使用 TIMESTAMPDIFF(DAY,NOW(),ac.ACTIVITY_BEGIN_DATE) = ' ' 这个条件

SELECT
        ap.QUARTER_ID , ap.OUT_APPLY_ID , ap.APPLY_TYPE , ap.REGION_ID , ap.REGION_NAME , ap.RES_TYPE ,
        ac.ACTIVITY_BEGIN_DATE , ac.ACTIVITY_END_DATE , ac.APPLY_TIME , ap.FILL_USER_NAME , ap.STATUS , 
        TIMESTAMPDIFF(DAY,NOW(),ac.ACTIVITY_BEGIN_DATE) AS CLOSE_DATE
        FROM out_apply ap
        LEFT JOIN activity ac
        ON ap.ACTIVITY_ID = ac.ACTIVITY_ID
        WHERE 1 = 1
        and ap.REGION_ID = '1'
        AND ap.RES_TYPE = '1'
        AND TIMESTAMPDIFF(DAY,NOW(),ac.ACTIVITY_BEGIN_DATE) = ''
        
        AND ap.APPLY_TYPE = '1'
        AND ap.QUARTER_ID = '1'
        AND ap.OUT_APPLY_NO = '1'
ORDER BY ap.ACTIVITY_ID;

愿编程让这个世界各美好