select employee_number, card_number FROM ( SELECT id, employee_number, card_number FROM ( SELECT case when LENGTH(pin) = 5 THEN CONCAT('000', pin) ELSE pin end as employee_number, cardno as card_number, '' AS id from gate_staff union ALL SELECT employee_number, card_number, id FROM gate_mdm_dm_cardinfo WHERE id in ( SELECT max(id) as maxId FROM gate_mdm_dm_cardinfo a INNER JOIN ( SELECT employee_number, MAX(last_update_time) max_date FROM gate_mdm_dm_cardinfo WHERE last_update_time <![CDATA[ >= ]]> #{startTime} AND last_update_time <![CDATA[ <= ]]> #{endTime} GROUP BY employee_number ) b ON a.employee_number = b.employee_number AND a.last_update_time = b.max_date GROUP BY a.employee_number) and status = 1 and public_flag = 'Y' ) as mid group by employee_number, card_number having count(*) = 1 ) AS ccc WHERE ccc.id != '' ； 这段代码什么意思

这段代码的意思是从两个表中选择员工号（employee_number）和卡号（card_number）。首先，在gate_staff表中，将pin字段长度为5的员工号前面补充三个0，如果不是五位数则保持不变，并将cardno字段作为卡号。然后，将gate_mdm_dm_cardinfo表中满足以下条件的记录加入结果集：记录的id是在子查询中找到的最大id，该子查询根据员工号分组并找到每个员工号的最大更新时间，然后将此记录与原表进行连接。满足条件的记录还需满足status为1且public_flag为'Y'。最后，根据员工号和卡号进行分组，并筛选出只有一条记录的结果。最后，在结果集中排除id为空的记录。

相关问题

select employee_number, card_number FROM ( SELECT id, employee_number, card_number FROM ( SELECT case when LENGTH(pin) = 5 THEN CONCAT('000', pin) ELSE pin end as employee_number, cardno as card_number, '' AS id from gate_staff union ALL SELECT employee_number, card_number, id FROM gate_mdm_dm_cardinfo WHERE id in ( SELECT max(id) as maxId FROM gate_mdm_dm_cardinfo a INNER JOIN ( SELECT employee_number, MAX(last_update_time) max_date FROM gate_mdm_dm_cardinfo WHERE last_update_time <![CDATA[ >= ]]> #{startTime} AND last_update_time <![CDATA[ <= ]]> #{endTime} GROUP BY employee_number ) b ON a.employee_number = b.employee_number AND a.last_update_time = b.max_date GROUP BY a.employee_number) and status = 1 and public_flag = 'Y' ) as mid group by employee_number, card_number having count(*) = 1 ) AS ccc WHERE ccc.id != ''; 优化这段代码

可以尝试对这段代码进行优化的几个方面：

1. 使用JOIN替代子查询：将gate_mdm_dm_cardinfo表中的子查询替换为JOIN操作，可以提高查询性能。将子查询中的条件移至ON子句中，以减少结果集大小。

SELECT employee_number, card_number
FROM (
    SELECT id, employee_number, card_number
    FROM (
        SELECT CASE
                WHEN LENGTH(pin) = 5 THEN CONCAT('000', pin)
                ELSE pin
            END AS employee_number,
            cardno AS card_number,
            '' AS id
        FROM gate_staff
        UNION ALL
        SELECT employee_number, card_number, id
        FROM gate_mdm_dm_cardinfo a
        INNER JOIN (
            SELECT employee_number, MAX(last_update_time) max_date
            FROM gate_mdm_dm_cardinfo
            WHERE last_update_time >= #{startTime} AND last_update_time <= #{endTime}
            GROUP BY employee_number
        ) b ON a.employee_number = b.employee_number AND a.last_update_time = b.max_date
        WHERE a.status = 1 AND a.public_flag = 'Y'
    ) AS mid
    GROUP BY employee_number, card_number
    HAVING COUNT(*) = 1
) AS ccc
WHERE ccc.id != '';

2. 添加索引：根据查询中涉及的字段，在gate_staff表和gate_mdm_dm_cardinfo表上添加适当的索引，以加快查询速度。

3. 优化查询计划：使用数据库的查询优化工具，如EXPLAIN语句，来分析查询计划并进行优化。根据分析结果，可以考虑调整查询的顺序、添加或删除索引等操作。

请注意，实际优化效果可能因数据库的特性和数据量而有所不同。建议在进行任何更改之前先备份数据库，并在测试环境中进行验证。

[Err] 1052 - Column 'employee_number' in field list is ambiguous 执行报错了怎么办

这个错误是因为查询中的字段名存在歧义，数据库无法确定是哪个表的字段。在你的查询中，涉及到多个表的employee_number字段，导致了这个错误。

为了解决这个问题，你可以在查询中明确指定每个字段的表名或表别名，以消除歧义。例如，假设gate_staff表的employee_number字段需要指定别名为"staff_employee_number"：

SELECT ccc.employee_number, ccc.card_number
FROM (
    SELECT mid.id, mid.employee_number, mid.card_number
    FROM (
        SELECT CASE
                WHEN LENGTH(pin) = 5 THEN CONCAT('000', pin)
                ELSE pin
            END AS employee_number,
            cardno AS card_number,
            '' AS id
        FROM gate_staff
        UNION ALL
        SELECT employee_number, card_number, id
        FROM gate_mdm_dm_cardinfo a
        INNER JOIN (
            SELECT employee_number, MAX(last_update_time) max_date
            FROM gate_mdm_dm_cardinfo
            WHERE last_update_time >= #{startTime} AND last_update_time <= #{endTime}
            GROUP BY employee_number
        ) b ON a.employee_number = b.employee_number AND a.last_update_time = b.max_date
        WHERE a.status = 1 AND a.public_flag = 'Y'
    ) AS mid
    GROUP BY mid.employee_number, mid.card_number
    HAVING COUNT(*) = 1
) AS ccc
WHERE ccc.id != '';

在这个例子中，我使用了ccc作为中间结果集的别名，并在最外层的SELECT语句中明确指定了字段的表名或别名。你可以根据实际情况对其他字段进行相似的处理。

请注意，根据你的表结构和需求，你可能需要适当调整别名或表名以匹配你的实际情况。