SQL中子查询的替代方案？

Question

我的查询应该返回几个列结果。还有一个要求，我必须为其中一列应用额外的过滤器。我找到了使用子查询（在主选择中选择）的解决方案，这里是示例：

SELECT 
   recid,
   price,
   receive_date,
   (SELECT deduction FROM Cost WHERE recid = c.recid AND status = 'Y') AS deduction
FROM Cost c
   INNER JOIN cost_types ct
      c.recid = ct.recid
WHERE cost_year = '2018'

上面的查询必须从Cost 表中提取与cost_type 表记录匹配的所有记录，其中cost_year 是2018。最重要的是，我必须提交deduction，其中status 是Y。我想知道我可以为这个查询使用什么其他技术？我知道UNION / UNION ALL，但那是更多的代码，似乎是多余的。如果有人有建议或更好的方法来处理这种情况，请告诉我。另外，我想知道这是否更适合存储过程？会推荐吗？

Answer 1

JOIN 是正常的方法，如下所示：

SELECT c.recid, c.price, c.receive_date,
       cd.deduction
FROM cost c INNER JOIN
     cost_types ct
     ON c.recid = ct.recid LEFT JOIN
     cost cd
     ON cd.recid = c.recid AND cd.status = 'Y'
WHERE c.cost_year = 2018;

这是猜测列的来源。如果列来自不同的表，请调整限定符。

你也可以使用窗口函数：

SELECT c.recid, c.price, c.receive_date,
       SUM(CASE WHEN c.status = 'Y' THEN c.deduction END) OVER (PARTITION BY c.recid) as deduction
FROM cost c INNER JOIN
     cost_types ct
     ON c.recid = ct.recid 
WHERE c.cost_year = 2018;

Answer 2

为什么不简单地使用CASE ？：

SELECT recid, price, receive_date,
      (CASE WHEN status = 'Y' THEN deduction END) AS deduction
FROM Cost c INNER JOIN 
     cost_types ct
     ON c.recid = ct.recid 
WHERE cost_year =  2018;

编辑：我怀疑你想要一些东西：

SELECT recid, price, receive_date, c1.deduction 
FROM Cost c INNER JOIN 
     cost_types ct
     ON c.recid = ct.recid OUTER APPLY 
     (SELECT TOP (1) c1.*
      FROM  Cost c1
      WHERE c1.recid = c.recid AND c1.status = 'Y'
      ORDER BY ?? -- Use ordering column based on you want deduction 
     ) c1;
WHERE cost_year =  2018;