MySQL的問題與工會/結合

Question

連接使用我有這兩個表：

表作者：

ID (INTEGER), 
author_name (VARCHAR), 
first_name (VARCHAR), 
last_name (VARCHAR), 
preferred_name (VARCHAR) 

表合着：

ID (INTEGER) 
author1ID (INTEGER), 
author2ID (INTEGER), 
paper_ID (INTEGER) // (ID of the co-authored paper referenced by this link) 

正如你可以看到，這兩個表有一個'ID'列，但它們不相關。

然而，當我使用這個查詢：

select author_name, count(*) as NumPapers from 
(select * from Author as a join CoAuthored as c on a.ID = c.author1ID 
union distinct 
select * from Author as b join CoAuthored as d on b.ID = d.author2ID) as t1 
group by author_name 
order by NumPapers; 

MySQL的gves我一個錯誤說：ERROR 1060 (42S21): Duplicate column name 'ID'

爲什麼會出現這種情況，如何避免呢？

Answer 1

而不是select * ...在正在聯合的兩個子查詢中使用select author_name ...。該問題源於具有ID列的Author和CoAuthored。 SELECT *帶來了這兩個列，MySQL不喜歡這些聯合使用這些產生具有兩個相同名稱列的結果集的想法。

Answer 2

試試這個：

select author_name, count(*) as NumPapers from 
(
    select a.id, a.author_name from Author as a 
    join CoAuthored as c on a.ID = c.author1ID 
    union distinct 
    select b.id, b.author_name from Author as b 
    join CoAuthored as d on b.ID = d.author2ID 
) as t1 
group by author_name 
order by NumPapers; 

因爲你不需要從合着的ID列，你可以只是沒有在內部查詢選擇它。這應該刪除重複的列錯誤，因爲它只選擇ID列中的一列。

Answer 3

怎麼樣：

SELECT author_name, COUNT(*) AS NumPapers 
    FROM Author AS a 
    JOIN CoAuthored AS c ON a.ID = c.author1ID OR a.ID = c.author2ID 
GROUP BY author_name 
ORDER BY NumPapers;