SELECT
 A.datelist AS pick_dt,
 IFNULL(B.pick_cnt, 0) pick_cnt,
 IFNULL(B.cust_cd, 0) cust_cd             //IFNULL(A,B) --如果结果不等于null,则返回A,等于null,就返回B
FROM
 calendar A
LEFT JOIN (
 SELECT
  sum(pick_cnt) AS pick_cnt,
  pick_dt,
  cust_cd
 FROM
  `crmkh_gp_bas_s_cust_pick_tmp`
 WHERE
  cust_cd = 1107607777
 GROUP BY
  pick_dt
 HAVING
  pick_dt BETWEEN ‘2018-11-02‘
 AND ‘2018-11-08‘
) B ON A.datelist = B.pick_dt
WHERE
 A.datelist BETWEEN ‘2018-11-02‘
AND ‘2018-11-08‘ 
原文:https://www.cnblogs.com/Darkqueen/p/9933418.html