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Question

我有一个查询，该查询将返回数据，假设members.language_id大于零并支持一种语言。 如果成员没有选择语言，则members.language_id默认为零，这在连接另一个表时无济于事（请记住，我没有设计此表结构，也无法更改）。

所以，我想做的是没有结果返回默认值时出现以下情况：
t.code = 'Kount_Declined' and t.language_id = 1

select m.email, t.subject, t.message from orders o
join members m using(member_id)
join languages l using(language_id)
join emailtempl t on t.language_id = GREATEST(l.language_id, 1)
where o.transaction_id = '3TK904KVMM8X'
and t.code = 'Kount_Declined'

有什么办法可以做到这一点？ 我了解可以使用2个查询来执行此操作，但是有没有其他查询的方法吗？

Answer 1

尝试修改的加入条件

    select m.email, t.subject, t.message
      from orders o
      join members m      using(member_id)
      join emailtempl t   on t.language_id = GREATEST(o.language_id, 1)
     where o.transaction_id = '3TK904KVMM8X'
       and t.code = 'Kount_Declined'
         ;

Answer 2

您可以使用IF

select m.email, t.subject, t.message
from orders o
join members m using(member_id)
join emailtempl t on o.language_id = IF(t.language_id = 0, 1, t.language_id)
where o.transaction_id = '3TK904KVMM8X'
and t.code = 'Kount_Declined';

Answer 3

由于@RyanVincent的建议我用union与limit 1 ，当时我能够拿出这一点，并已通过了所有，我一直在寻找完成这一壮举的测试。

(
    select t.language_id, m.email, t.subject, t.message from orders o
    join members m using(member_id)
    join languages l using(language_id)
    join emailtempl t using(language_id)
    where o.transaction_id = '3TK904KVMM8X'
    and t.code = 'Kount_Declined' limit 1
)
union
(
    select t.language_id, m.email, t.subject, t.message from orders o
    join members m using(member_id)
    join emailtempl t on t.language_id = 1
    where o.transaction_id = '3TK904KVMM8X'
    and t.code = 'Kount_Declined' limit 1
)
limit 1