Question

我有这两个表：

表作者：

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

表格CoOuthored：

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

如您所见，两个表都有一个列'ID'，但它们是不相关的。

但是，当我使用此查询时：

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给我一个错误说：ERROR 1060 (42S21): Duplicate column name 'ID'

为什么会发生这种情况，我该如何避免呢？

Answer 1

而不是select * ...在联合的两个子查询中使用select author_name ...。问题源于Author和CoAuthored都有ID列。 SELECT *带来了这两个列，MySQL不喜欢UNIONing这些产生带有两个同名列的结果集的想法。

Answer 2

试试这个：

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;

由于您不需要CoAuthored中的ID列，因此您无法在内部查询中选择它。这应该删除您的重复列错误，因为它只选择了1个ID列。

Answer 3

怎么样：

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;

与union / join一起使用的mysql问题

3 个答案: