mysql的联合/连接问题

Question

我有这两张表：

表作者：

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

表共同创作：

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“ID”列，但它们不相关。

但是，当我使用这个查询时：

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

为什么会发生这种情况，我该如何避免？

Answer 1

而不是select * ... 在被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 不喜欢将这些合并以生成具有两个同名列的结果集。

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;

由于您不需要来自 CoAuthored 的 ID 列，因此您不能在内部查询中选择它。这应该会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;