根据同一表的不同条件返回列

Question

鉴于下表，我希望能够运行一个查询，该查询将返回两列：一列用于哺乳动物，另一列用于其他所有列。

id class 
--------- 
1 reptile 
2 bird 
3 mammal 
4 mammal 
5 mammal 

预期输出：

mammal others 
-------------- 
3  1 
4  2 
5  

我曾尝试以下（除其他奇怪的事情），但得到的MS Access中的“不能从子句重复的表名......在”错误：

SELECT a AS mammals, b AS others 
FROM 
(SELECT id AS a FROM animals WHERE class = "mammal"), 
(SELECT id AS b FROM animals WHERE class <> "mammal"); 

在SQL这可能吗？谢谢！

Answer 1

你需要生成unique号码都查询class='mammal'和class<>'mammal'然后使用唯一编号加入两个查询。由于在class='mammal'和class<>'mammal'记录的数量可能会有所不同，我们需要使用FULL OUTER JOIN

，但是这将是一个痛苦这样做在Ms-Access不支持Row_Number和Full Outer Join

SELECT * 
FROM (SELECT a.id, 
       a.class, 
       Count(*) AS Rn 
     FROM animals a 
       INNER JOIN animals b 
         ON a.id >= b.id 
     WHERE b.class = 'mammal' 
     GROUP BY a.id, 
        a.class) a 
     LEFT JOIN (SELECT a.id, 
         a.class, 
         Count(*) AS Rn 
        FROM animals a 
         INNER JOIN animals b 
           ON a.id >= b.id 
        WHERE a.class <> 'mammal' 
        GROUP BY a.id, 
          a.class) b 
       ON a.rn = b.rn 
UNION ALL 
SELECT * 
FROM (SELECT a.id, 
       a.class, 
       Count(*) AS Rn 
     FROM animals a 
       INNER JOIN animals b 
         ON a.id >= b.id 
     WHERE b.class = 'mammal' 
     GROUP BY a.id, 
        a.class) a 
     RIGHT JOIN (SELECT a.id, 
          a.class, 
          Count(*) AS Rn 
        FROM animals a 
          INNER JOIN animals b 
            ON a.id >= b.id 
        WHERE a.class <> 'mammal' 
        GROUP BY a.id, 
          a.class) b 
       ON a.rn = b.rn 
WHERE a.rn IS NULL 

注意：我不太确定Ms-Access的语法可能是我们需要围绕连接的泛式分析，但是这应该会给你一个启动的想法

Answer 2

尝试这可能有助于

select id as Mammal , null as Others from table where class='mammal' 
union 
select null as Mammal , id as Others from table where class<>'mammal'