Uso di LAG() e LEAD()

Analisi delle serie temporali in SQL Server

Maham Faisal Khan

Senior Data Science Content Developer

La funzione finestra LAG()

SELECT
    dsr.CustomerID,
    dsr.MonthStartDate,
    LAG(dsr.NumberOfVisits) OVER (PARTITION BY dsr.CustomerID ORDER BY dsr.MonthStartDate) AS Prior,
    dsr.NumberOfVisits
FROM dbo.DaySpaRollup dsr;

CustomerID	MonthStartDate	Prior	NumberOfVisits
1	2018-12-01	NULL	49
1	2019-01-01	49	117
1	2019-02-01	117	104

La funzione finestra LEAD()

SELECT
    dsr.CustomerID,
    dsr.MonthStartDate,
    dsr.NumberOfVisits,
    LEAD(dsr.NumberOfVisits) OVER (PARTITION BY dsr.CustomerID ORDER BY dsr.MonthStartDate) AS Next
FROM dbo.DaySpaRollup dsr;

CustomerID	MonthStartDate	NumberOfVisits	Next
1	2018-12-01	49	117
1	2019-01-01	117	104
1	2019-02-01	104	108

Specificare il numero di righe precedenti

SELECT
   dsr.CustomerID,
   dsr.MonthStartDate,
   LAG(dsr.NumberOfVisits, 2) OVER (PARTITION BY dsr.CustomerID ORDER BY dsr.MonthStartDate) AS Prior2,
   LAG(dsr.NumberOfVisits, 1) OVER (PARTITION BY dsr.CustomerID ORDER BY dsr.MonthStartDate) AS Prior1,
   dsr.NumberOfVisits
FROM dbo.DaySpaRollup dsr;

CustomerID	MonthStartDate	Prior2	Prior	NumberOfVisits
1	2018-12-01	NULL	NULL	49
1	2019-01-01	NULL	49	117
1	2019-02-01	49	117	104

Finestre e filtri

SELECT
    Date,
    LAG(Val, 1) OVER(ORDER BY DATE) AS PriorVal,
    Val
FROM t;

Date	PriorVal	Val
2019-01-01	NULL	3
2019-01-02	3	6
2019-01-03	6	4

SELECT
    Date,
    LAG(Val, 1) OVER(ORDER BY DATE) AS PriorVal,
    Val
FROM t
WHERE
    t.Date > '2019-01-02';

Date	PriorVal	Val
2019-01-03	NULL	4

Finestre, filtri e CTE

WITH records AS (
  SELECT
      Date,
      LAG(Val, 1) OVER(ORDER BY Date) AS PriorVal,
      Val
  FROM t
)
SELECT
    r.Date,
    r.PriorVal,
    r.Val
FROM records r
WHERE
    r.Date > '2019-01-02';

Date	PriorVal	Val
2019-01-03	6	4

Passons à la pratique !

Analisi delle serie temporali in SQL Server