ORA-00932：不一致的數據類型：預計 - 在嘗試連接不同的表時遇到BLOB

Question

嘗試將JOIN語句用於從多個表中選擇值時，我得到ORA-00932: inconsistent datatypes: expected - got BLOB。以下是我的Oracle SQL代碼：

SELECT 
    MAX(questions.id), 
    MAX(questions.question), 
    MAX(questions.author), 
    MAX(questions.datetime), 
    MAX(answers.answer), 
    MAX(answers.usr), 
    MAX(answers.ansdatetime) 
FROM 
    questions 
LEFT JOIN 
    answers ON 
     questions.id = answers.question 
LEFT JOIN 
    questions_tags ON 
     questions.id = questions_tags.question_id 
WHERE 
    questions_tags.tag_id IN (1,2,3,4,5,6) 
GROUP BY 
    questions.id, answers.id 
ORDER BY 
    questions.datetime DESC 

以下是三個表的表結構：

questions：

id int not null, 
question varchar(999), 
details varchar(1000) not null, 
author int not null, 
datetime varchar(999) 

answers

id int not null, primary key(id), 
question int not null, 
answer blob not null, 
usr int not null, 
ansdatetime int not null 

questions_tags

id INT NOT NULL, 
question_id INT NOT NULL, 
tag_id INT NOT NULL, 
PRIMARY KEY(id) 

這裏有什麼問題？

Answer 1

正如評論所說，這不起作用，因爲您不能在blob列上執行max列。然而，你可以重寫這個查詢，以不使用聚合：

with a as (
    select 
     *, -- might need to enumerate columns 
     row_number() over (partition by question order by ansdatetime desc) as rn 
    from 
     answers 
) 
select 
    q.id, 
    q.question, 
    q.author, 
    q.datetime, 
    a.answer, 
    a.usr, 
    a.ansdatetime 
from 
    questions q 
     left join 
    a 
     on q.id = a.question and rn = 1 -- assuming you're only looking for latest answer per q 
where 
    exists (
     select 
      'x' 
     from 
      questions_tags t 
     where 
      q.id = t.question_id and 
      t.tag_id in (1,2,3,4,5,6) 
    ) 
order by 
    q.datetime desc