有条件地将字段添加到查询的输出

Question

仅在user_answer.user_id = 10时才希望选择user_answer.status

我使用此SQL查询从多个表（ 问题 ， q_t ， 标签 ， user_answer ）返回结果

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select 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

如何添加此条件？

谢谢，

Answer 1

您可以使用CASE WHEN来做到这一点：

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);

如果您希望将状态应用于所有问题，则可以使用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);

Answer 2

select question.text,group_concat(tag.text), count(user_answer.question_id) as tt
    ,if((user_answer.id=10),(select user_answer.status),(''))as status
    from question`enter code here`
    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)