案例研究 UDFs

在 SQL Server 中撰寫函式與預存程序

Meghan Kwartler

IT Consultant

計程車營運問題

歐洲私募股權公司尋找美國運輸業投資機會。
每個星期的每一天，紐約市各行政區的「每距離平均車資」、「乘車次數」與「總乘車時間」是多少？

黃色計程車照片

計程車營運問題

歐洲私募股權公司尋找美國運輸業投資機會。
每個星期的每一天，紐約市各行政區的「每距離平均車資」、「乘車次數」與「總乘車時間」是多少？
在各行政區內，哪些「上車地點」應排入各駕駛「班別」？

黃色計程車照片

單位換算 UDFs

CREATE FUNCTION dbo.ConvertMileToMeter (@miles numeric)
RETURNS numeric
AS
BEGIN
RETURN (SELECT @miles * 1609.34)
END

CREATE FUNCTION dbo.ConvertCurrency (@Currency numeric, @ExchangeRate numeric)
RETURNS numeric
AS
BEGIN
RETURN (SELECT @ExchangeRate * @Currency)
END

SELECT TripDistance as 'MileDistance', 
dbo.ConvertMileToMeter (TripDistance) as 'MeterDistance', 
FareAmount as 'FareUSD', 
dbo.ConvertCurrency (FareAmount, '.78') as 'FareGBP' 
FROM dbo.YellowTripData

+--------------+---------------+---------+---------+
| MileDistance | MeterDistance | FareUSD | FareGBP |
|--------------+---------------+---------+---------+
|  1.10        | 1609          |  6      | 6       |
|  0.02        | 0             | 52      | 52      |
|  0.50        | 1609          |  4      | 4       |
|  7.75        | 12875         | 22      | 22      |
|  0.80        | 1609          |  6      | 6       |
|  0.90        | 1609          |  7      | 7       |
|  1.76        | 3219          |  7      | 7       |
|  8.47        | 12875         | 24      | 24      |
|  2.40        | 3219          | 10.50   | 11      |
| 12.60        | 20921         | 60      | 60      |
|  0.90        | 1609          |  7      | 7       |
+--------------+---------------+---------+---------+

反覆調整

ALTER FUNCTION dbo.ConvertMileToMeter (
  @miles numeric (18, 2)
) RETURNS numeric (18, 2) as BEGIN RETURN (
  SELECT @miles * 1609.34
) END;

ALTER FUNCTION dbo.ConvertCurrency (
  @Currency numeric (18, 2), 
  @ExchangeRate numeric(18, 2)
) RETURNS numeric (18, 2) AS BEGIN RETURN (
  SELECT @ExchangeRate * @Currency
) END;

班別怎麼分？

CREATE FUNCTION dbo.GetShift (@Hour int)
RETURNS int
AS
BEGIN
RETURN (CASE 
    WHEN @Hour >= 0 AND @Hour < 9 THEN 1
    WHEN @Hour >= 9 AND @Hour < 18 THEN 2
    WHEN @Hour >= 18 AND @Hour < 24 THEN 3
END)
END;

測試班別

SELECT 
  DATENAME(hour, PickupDate) AS 'Hour', 
  dbo.GetShift (
    DATENAME(hour, PickupDate)
  ) AS 'Shift' 
FROM YellowTripData 
GROUP BY DATENAME(hour, PickupDate) 
ORDER BY 
  dbo.GetShift (
    DATENAME(hour, PickupDate)
  )

+------+-------+
| Hour | Shift |
|------+-------|
|  3   | 1     |
|  6   | 1     |
|  4   | 1     |
|  2   | 1     |
|  0   | 1     |
|  8   | 1     |
|  5   | 1     |
|  1   | 1     |
|  7   | 1     |
|  9   | 2     |
| 11   | 2     |
| 15   | 2     |
| 14   | 2     |
| ...  | ...   |
+------+-------|

換你試試！

在 SQL Server 中撰寫函式與預存程序