COUNT返回null时如何插入零？ MySQL

Question

我在触发器中有以下内容，并希望它在返回 NULL 时插入零。在过去的两天里，我在 Stackoverflow 和其他网站上阅读了各种帖子，但一直无法实现我想要的……并且恐怕这不可能……？此查询返回 MCVMoodle.mdl_facetoface_signups.userid 中的所有用户，其计数值为等级（满足 WHERE 条件），计数值为 1 或更多，但没有 0/null。我理解在没有结果/null 的情况下工作的困难，但是当计数为 0/null 时是否有插入零？

INSERT INTO MCVMoodle.mdl_user_info_data_f2fskipcount (userid, fieldid, datab)

SELECT MCVMoodle.mdl_facetoface_signups.userid as 'userid', 4 as 'fieldid',
      COUNT(MCVMoodle.mdl_facetoface_signups_status.grade) as 'datab'
        FROM
            MCVMoodle.mdl_facetoface_signups
        LEFT JOIN
            MCVMoodle.mdl_facetoface_signups_status on MCVMoodle.mdl_facetoface_signups.id = MCVMoodle.mdl_facetoface_signups_status.signupid
       WHERE
    MCVMoodle.mdl_facetoface_signups_status.statuscode = 80
      AND  MCVMoodle.mdl_facetoface_signups_status.superceded = 0
      AND  MCVMoodle.mdl_facetoface_signups_status.grade = 0
       GROUP BY
        MCVMoodle.mdl_facetoface_signups.userid;

以下是最终对我有用的查询。感谢 Gordon 在这方面提供的重要帮助。

INSERT INTO MCVMoodle.mdl_user_info_data_f2fskipcount (userid, fieldid, datab)

SELECT MCVMoodle.mdl_facetoface_signups.userid as 'userid', 4 as 'fieldid',
             COUNT(MCVMoodle.mdl_facetoface_signups_status.grade) as 'datab'
      FROM MCVMoodle.mdl_facetoface_signups LEFT JOIN
           MCVMoodle.mdl_facetoface_signups_status
           on MCVMoodle.mdl_facetoface_signups.id = MCVMoodle.mdl_facetoface_signups_status.signupid and
              MCVMoodle.mdl_facetoface_signups_status.statuscode = 80 AND
              MCVMoodle.mdl_facetoface_signups_status.superceded = 0 AND
              MCVMoodle.mdl_facetoface_signups_status.grade = 0
      GROUP BY MCVMoodle.mdl_facetoface_signups.userid
      ON DUPLICATE KEY UPDATE
       datab=VALUES(datab);

Answer 1

如果我理解正确，您想插入查询中的所有内容。但是如果查询为空，则插入一行所有NULLs。以下是这样做的，但不是在 MySQL 中：

with cte as (
      SELECT MCVMoodle.mdl_facetoface_signups.userid as 'userid', 4 as 'fieldid',
             COUNT(MCVMoodle.mdl_facetoface_signups_status.grade) as 'datab'
      FROM MCVMoodle.mdl_facetoface_signups LEFT JOIN
           MCVMoodle.mdl_facetoface_signups_status
           on MCVMoodle.mdl_facetoface_signups.id = MCVMoodle.mdl_facetoface_signups_status.signupid
      WHERE MCVMoodle.mdl_facetoface_signups_status.statuscode = 80 AND
            MCVMoodle.mdl_facetoface_signups_status.superceded = 0 AND
            MCVMoodle.mdl_facetoface_signups_status.grade = 0
      GROUP BY MCVMoodle.mdl_facetoface_signups.userid
     )
INSERT INTO MCVMoodle.mdl_user_info_data_f2fskipcount (userid, fieldid, datab)
    select *
    from cte
    union all
    select NULL, NULL, NULL
    where not exists (select * from CTE);

MySQL版本比较复杂：

INSERT INTO MCVMoodle.mdl_user_info_data_f2fskipcount (userid, fieldid, datab)
    select *
    from (SELECT MCVMoodle.mdl_facetoface_signups.userid as 'userid', 4 as 'fieldid',
                 COUNT(MCVMoodle.mdl_facetoface_signups_status.grade) as 'datab'
          FROM MCVMoodle.mdl_facetoface_signups LEFT JOIN
               MCVMoodle.mdl_facetoface_signups_status
               on MCVMoodle.mdl_facetoface_signups.id = MCVMoodle.mdl_facetoface_signups_status.signupid
          WHERE MCVMoodle.mdl_facetoface_signups_status.statuscode = 80 AND
                MCVMoodle.mdl_facetoface_signups_status.superceded = 0 AND
                MCVMoodle.mdl_facetoface_signups_status.grade = 0
          GROUP BY MCVMoodle.mdl_facetoface_signups.userid
         ) cte
    union all
    select NULL, NULL, NULL
    where not exists (SELECT MCVMoodle.mdl_facetoface_signups.userid as 'userid', 4 as 'fieldid',
                             COUNT(MCVMoodle.mdl_facetoface_signups_status.grade) as 'datab'
                      FROM MCVMoodle.mdl_facetoface_signups LEFT JOIN
                           MCVMoodle.mdl_facetoface_signups_status
                           on MCVMoodle.mdl_facetoface_signups.id = MCVMoodle.mdl_facetoface_signups_status.signupid
                      WHERE MCVMoodle.mdl_facetoface_signups_status.statuscode = 80 AND
                            MCVMoodle.mdl_facetoface_signups_status.superceded = 0 AND
                            MCVMoodle.mdl_facetoface_signups_status.grade = 0
                      GROUP BY MCVMoodle.mdl_facetoface_signups.userid
            );

编辑：

在我看来，您可能只想包含查询中的所有组合，即使是那些被where 子句消除的组合。如果是，则将条件移至on 子句：

with cte as (
      SELECT MCVMoodle.mdl_facetoface_signups.userid as 'userid', 4 as 'fieldid',
             COUNT(MCVMoodle.mdl_facetoface_signups_status.grade) as 'datab'
      FROM MCVMoodle.mdl_facetoface_signups LEFT JOIN
           MCVMoodle.mdl_facetoface_signups_status
           on MCVMoodle.mdl_facetoface_signups.id = MCVMoodle.mdl_facetoface_signups_status.signupid and
              MCVMoodle.mdl_facetoface_signups_status.statuscode = 80 AND
              MCVMoodle.mdl_facetoface_signups_status.superceded = 0 AND
              MCVMoodle.mdl_facetoface_signups_status.grade = 0
      GROUP BY MCVMoodle.mdl_facetoface_signups.userid
     )
INSERT INTO MCVMoodle.mdl_user_info_data_f2fskipcount (userid, fieldid, datab)
    select *
    from cte;