有條件地將字段添加到查詢的輸出

Question

僅在user_answer.user_id = 10時才希望選擇user_answer.status

我使用此SQL查詢從多個表（ 問題 ， q_t ， 標簽 ， user_answer ）返回結果

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)

結果：

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

我需要從user_answer表中添加新的select user_answer.status ，並附帶條件（僅適用於檢索此選擇）：

select user_answer.status    
where user_answer.user_id = 10

如何添加此條件？

謝謝，

Answer 1

您可以使用CASE WHEN來做到這一點：

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);

如果您希望將狀態應用於所有問題，則可以使用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)