SQL子查询以获取第一条记录

Question

我需要执行类似下面的查询。

SELECT TO_CHAR(ROWNUM),
A.Name,
B.Order,
(SELECT * FROM (
SELECT ROUND(LAST_ORDER_AMOUNT,5) FROM ORDERS WHERE ID=A.id AND REQUEST_LEVEL='N' ORDER BY O_DATE DESC)
WHERE ROWNUM =1) AS AMOUNT
FROM Table1 A LEFT JOIN Table2 B
ON A.TYPE_CODE = B.ENTITY_TYPE

但这给了我A.ID在oracle中是无效错误。 我需要从内部查询中获取第一条记录，因为它将返回多个记录。

有人可以让我知道如何绑定这些表以实现我的目标。

先感谢您。

Answer 1

您可以使用WITH子句重写子查询，但语法不完全确定，但应类似于以下内容。

WITH    AmountQuery
      AS (
           SELECT ID
               ,ROUND(LAST_ORDER_AMOUNT, 5) AS AmountValue
               ,ROW_NUMBER() OVER ( ORDER BY O_DATE DESC ) AS RN
            FROM ORDERS
            WHERE REQUEST_LEVEL = 'N'
         )
SELECT TO_CHAR(ROWNUM)
       ,A.Name
       ,B.Order
       ,C.AmountValue
    FROM Table1 A
    LEFT JOIN Table2 B
        ON A.TYPE_CODE = B.ENTITY_TYPE
    LEFT JOIN AmountQuery C
        ON a.ID = c.ID
           AND c.RN = 1

这是SQLFiddle来显示其工作原理。

http://sqlfiddle.com/#!4/696b6/36

Answer 2

LIMIT可能为您完成了从子查询中选择一条记录的工作（它在MySQL中对我有用。我没有Oracle，但我认为它可能相似）。 尝试这样的事情：

SELECT TO_CHAR(ROWNUM),
A.Name,
B.Order,
COALESCE( C.AMOUNT ) as AMOUNT,
FROM Table1 A LEFT JOIN Table2 B
ON A.TYPE_CODE = B.ENTITY_TYPE
LEFT JOIN ( SELECT ROUND(LAST_ORDER_AMOUNT,5) AS AMOUNT FROM ORDERS WHERE REQUEST_LEVEL='N' ORDER BY  O_DATE DESC ) C ON C.ID = A.id
group by A.id;