我执行一个查询，它给我的结果是4，5，6。如果我直接在where in子句中的查询b中使用这个结果，它会再次给出完美的结果。但是如果我在查询c中同时使用这两个查询，它不会生成任何答案。
查询a

SELECT `region`.`district_id` 
FROM `region` 
LEFT JOIN `regional_owner` ON `regional_owner`.`r_id` = `region`.`id`
WHERE `regional_owner`.`email_id` = 'balajibarhate10@gmail.com';

产生o/p-4,5,6，我在下面使用
查询b

SELECT sum(`purchase`.`purchase_ammount`), `franchise`.`district_id`
FROM `purchase`
LEFT JOIN `franchise` ON `franchise`.`id` = `purchase`.`f_id`
WHERE `franchise`.`district_id` in ( 
         4,5,6
    )
    AND purchase.purchase_date BETWEEN Date_format(CURDATE(), "%Y-%m-01") AND CURDATE()
GROUP BY `franchise`.`district_id`

现在，我将上述两个查询合并为一个查询，如下所示
查询c

SELECT sum(`purchase`.`purchase_ammount`), `franchise`.`district_id`
FROM `purchase`
LEFT JOIN `franchise` ON `franchise`.`id` = `purchase`.`f_id`
WHERE `franchise`.`district_id` in ( 
    SELECT `region`.`district_id` 
    FROM `region` 
    LEFT JOIN `regional_owner` ON `regional_owner`.`r_id` = `region`.`id`
    WHERE `regional_owner`.`email_id` = 'balajibarhate10@gmail.com'
)

AND purchase.purchase_date BETWEEN Date_format(CURDATE(), "%Y-%m-01") AND CURDATE()
GROUP BY `franchise`.`district_id`

我不明白，哪一部分错了。请引导我。