mysql - 有条件地将字段添加到查询的输出

Question

仅当 user_answer.user_id = 10 时才选择 user_answer.status

我使用此 SQL 查询从多个表返回结果(question、q_t、tag、user_answer)

SQL:

select question.text,group_concat(tag.text), count(user_answer.question_id) as tt
from question
left join q_t on question.id = q_t.wall_id
left join user_answer on question.id = user_answer.question_id
left join tag on q_t.tag_id = tag.id
where question.id in (1000001,1000002,1000003,1000004,1000005) 
group by  question.text
order by field(question.id,1000001,1000002,1000003,1000004,1000005)

结果:

text                         text                  tt
where is England?         Geography,Continent      33
how many ...?             sport,Europe              2

我需要从 user_answer 表中添加新的选择 user_answer.status，并附带条件(仅适用于检索此选择):

select user_answer.status    
where user_answer.user_id = 10

如何添加这个条件？

谢谢

Answer 1

您可以使用 CASE WHEN 来执行此操作:

select 
  question.text,group_concat(tag.text), 
  count(user_answer.question_id) as tt, 
  CASE WHEN user_answer.user_id = 10 THEN user_answer.status ELSE NULL END as status
from question
left join q_t on question.id = q_t.wall_id
left join user_answer on question.id = user_answer.question_id
left join tag on q_t.tag_id = tag.id
where question.id in (1000001,1000002,1000003,1000004,1000005) 
group by  
  question.text,  
  CASE WHEN user_answer.user_id = 10 THEN user_answer.status ELSE NULL END
order by field(question.id,1000001,1000002,1000003,1000004,1000005);

如果您希望状态应用于所有问题，您可以使用 MAX:

select 
  question.text,group_concat(tag.text), 
  count(user_answer.question_id) as tt, 
  MAX(CASE 
        WHEN user_answer.user_id = 10 
        THEN user_answer.status 
        ELSE NULL 
       END) as status
from question
left join q_t on question.id = q_t.wall_id
left join user_answer on question.id = user_answer.question_id
left join tag on q_t.tag_id = tag.id
where question.id in (1000001,1000002,1000003,1000004,1000005) 
group by  
  question.text
order by field(question.id,1000001,1000002,1000003,1000004,1000005);