我已经一个sql script
在one table
用于与two alias
,并作出JOIN
它 但我需要理解为什么以及何时需要它这样做 请详细说明?为什么要使用相同表的别名,做自连接
SELECT
ISNULL(MarketValue,0)
FROM
(
SELECT TOP 5
Delta,
MarketValue
FROM
(
SELECT DISTINCT
FormatNumber(SUM([CURRENT].MarkToMarket), 0, ',', 0) AS Delta,
AVG([CURRENT].Mark) * SUM([CURRENT].Position) AS NumericPosition,
FormatNumber(AVG([CURRENT].Mark) * SUM([CURRENT].Position)* CASE WHEN [CURRENT].SecurityType IN ('Equity','Equity Option') THEN 100 ELSE 1 END, 0, ',', NULL) AS MarketValue
FROM
NAV [CURRENT]
LEFT JOIN
NAV Compare ON
[CURRENT].PurchaseLotId = Compare.PurchaseLotId AND
Compare.Date = '2012-06-06'
WHERE
[CURRENT].SecurityType not in ('Equity')
GROUP BY
ORDER BY
NumericDelta DESC
) Movers
FOR XML PATH ('TR'), TYPE
) AS VARCHAR(MAX)
)
Why it is used like FROM
NAV [CURRENT]
LEFT JOIN
NAV Compare ON
[CURRENT].PurchaseLotId = Compare.PurchaseLotId AND
Compare.Date = '2012-06-06'
NAV是只有一个表又是什么做采取同桌NAV(当前)和NAV比较和使用左连接两个别名的目的是什么?
请详细说明一下。