Question

我有三个表：电子邮件， customer_new 和 customers_old 。现在，在电子邮件表中，有一个名为类型的字段，其中包含 0 或 1 。因此，如果类型包含 0 ，那么我想从 customer_old 表中 customer_old <获取名称列/ strong>表。

为此我在mysql查询下面做了：

SELECT email.*, CASE email.`type` WHEN 1 THEN customer_old.name ELSE customer_new.name END AS customer_name LEFT JOIN CASE email.`type` WHEN 0 THEN customer_new ON email.customer_id = customer_new.id ELSE customer_old ON email.customer_id = customer_old.id END FROM email

现在，当我运行此查询时，mysql总是抛出错误有人可以通过基于条件的CASE语句解决我关于加入两个表的问题吗？

提前致谢。

Answer 1

尝试以下查询。

SELECT email . * , IF( email.type = '1', customer_old.name, customer_new.name ) AS name 
FROM email
LEFT JOIN customer_old ON ( customer_old.id = email.customer_id ) 
LEFT JOIN customer_new ON ( customer_new.id = email.customer_id )

Answer 2

select email.*,
CASE email.type 
  when 1 then customer_old.name
  else customer_new.name 
  end as filteredname
from email,customer_new,customer_old 
where 
  customer_new.id=email.id 
and 
  customer_old.id=email.id;

您可以使用左连接来优化执行。

Answer 3

另一种选择，

SELECT * FROM email 
LEFT JOIN customer_new AS customer ON email.customer_id = customer.id 
WHERE email.type = 0

UNION

SELECT * FROM email 
LEFT JOIN customer_old AS customer ON email.customer_id = customer.id 
WHERE email.type = 1

如何在JOIN语句中使用CASE根据条件连接两个表？

3 个答案: