如何从子查询(在SQL Server中)中选择多个列,这些列应该为主查询中的每个记录创建一条记录(选择前1)？

Question

我知道我可以使用以下语法从子查询中选择一列:

SELECT A.SalesOrderID, A.OrderDate,
       (
       SELECT TOP 1 B.Foo
       FROM B
       WHERE A.SalesOrderID = B.SalesOrderID
       ) AS FooFromB
FROM A
WHERE A.Date BETWEEN '2000-1-4' AND '2010-1-4'

但是使用子查询中的多个列的正确语法是什么(在我的例子中是select top 1子查询)？非常感谢你.

Answer 1

这里通常是如何从子查询中选择多个列:

SELECT
     A.SalesOrderID,
     A.OrderDate,
     SQ.Max_Foo,
     SQ.Max_Foo2
FROM
     A
LEFT OUTER JOIN
     (
     SELECT
          B.SalesOrderID,
          MAX(B.Foo) AS Max_Foo,
          MAX(B.Foo2) AS Max_Foo2
     FROM
          B
     GROUP BY
          B.SalesOrderID
     ) AS SQ ON SQ.SalesOrderID = A.SalesOrderID

如果你最终想要做的是从Foo的最高值的行中获取值(而不是Foo的最大值和Foo2的最大值 - 这不是同一个东西),那么以下通常会比子查询:

SELECT
     A.SalesOrderID,
     A.OrderDate,
     B1.Foo,
     B1.Foo2
FROM
     A
LEFT OUTER JOIN B AS B1 ON
     B1.SalesOrderID = A.SalesOrderID
LEFT OUTER JOIN B AS B2 ON
     B2.SalesOrderID = A.SalesOrderID AND
     B2.Foo > B1.Foo
WHERE
     B2.SalesOrderID IS NULL

你基本上是在说,给我B行,我找不到B中的任何其他行,具有相同的SalesOrderID和更大的Foo.

Answer 2

SELECT a.salesorderid, a.orderdate, s.orderdate, s.salesorderid
FROM A a
OUTER APPLY (SELECT top(1) *
             FROM B b WHERE a.salesorderid = b.salesorderid) as s
WHERE A.Date BETWEEN '2000-1-4' AND '2010-1-4'

Answer 3

你必须加入:

SELECT A.SalesOrderID, B.Foo
FROM A
JOIN B bo ON bo.id = (
     SELECT TOP 1 id
     FROM B bi
     WHERE bi.SalesOrderID = a.SalesOrderID
     ORDER BY bi.whatever
     )
WHERE A.Date BETWEEN '2000-1-4' AND '2010-1-4'

,假设b.id是PRIMARY KEY开启的B

在MS SQL 2005和更高的你可以使用这个语法:

SELECT SalesOrderID, Foo
FROM (
  SELECT A.SalesOrderId, B.Foo,
         ROW_NUMBER() OVER (PARTITION BY B.SalesOrderId ORDER BY B.whatever) AS rn
  FROM A
  JOIN B ON B.SalesOrderID = A.SalesOrderID
  WHERE A.Date BETWEEN '2000-1-4' AND '2010-1-4'
) i
WHERE rn

这将从B每个记录中精确选择一条记录SalesOrderId.

Answer 4

select t1.*, sq.*
from table1 t1,
   (select a,b,c from table2 ...) sq
where ...