Match between tables with New, Not New - sql

Question

I am still learning TSQL at the moment and im new to here so forgive me if Ive not done this right.
I have a table that each day loads new days data. Each day that loads has a report date for the previous day.
I want to get yesterdays data (eg - 17/09/2019) from the table, and I want to look at the data in the same table from the day before that (eg - 16/09/2019) and I want to run a check for the reference number and if the Reference number appears on the day before then I want it to say Not New, and if it does match to the day before then I want it to say New.
The columns I have is :
ReferenceNumber, ReportData, NewAppt
NewAppt column will be where it put the outcome of New/Not New

Answer 1

Something like this should work:
WITH Yesterday AS (
SELECT DISTINCT
ReferenceNumber,
CONVERT(DATE, ReportDate) AS ReportDate
FROM
MyTable
WHERE
CONVERT(DATE, ReportDate) = CONVERT(DATE, DATEADD(DAY, -1, GETDATE())),
DayBeforeYesterday AS (
SELECT DISTINCT
ReferenceNumber
FROM
MyTable
WHERE
CONVERT(DATE, ReportDate) = CONVERT(DATE, DATEADD(DAY, -2, GETDATE()))
SELECT
y.ReferenceNumber,
y.ReportDate,
CASE
WHEN x.ReferenceNumber IS NOT NULL THEN 0
ELSE 1
END AS NewAppointment
FROM
Yesterday y
LEFT JOIN DayBeforeYesterday x ON x.ReferenceNumber = y.ReferenceNumber;
Make a list of all the DISTINCT reference numbers from each day, and then join them into one big list, with the logic to see if there was a reference number yesterday that was also in the list from the day before yesterday.

Answer 2

I suppose your column ReportData is some sort of 'date' type and contains only the date (no time).
Furthermore, for each date, there should be at most 1 record for a specific ReferenceNumber.
In that case, try this:
SELECT t1.ReferenceNumber,
t1.ReportData,
CASE
WHEN t2.ReferenceNumber IS NULL THEN 'New'
ELSE 'Not New'
END AS NewAppt
FROM my_table t1
LEFT OUTER JOIN my_table t2
ON t1.ReferenceNumber = t2.ReferenceNumber
AND t2.ReportData = DATEADD(day, -1, t1.ReportData)
WHERE t1.ReportData = '2019-09-17';

Answer 3

Here's an approach using LAG which removes the need to join to the same table several times and instead just checks the preceding row for that Reference Number.
Note that in my interpretation of your request if a Reference Number disappears for a day and then returns then it's flagged as new. You can adapt the query to simply check if the number has appeared at any point in the past if that's not what you need.
CREATE TABLE #TestData (ReferenceNumber int,Reportdata date)
INSERT INTO #TestData
VALUES (1,'2019-01-16'),(1,'2019-01-17'),(1,'2019-01-18'),(2,'2019-01-18'),(3,'2019-01-17'),(3,'2019-01-18'),(4,'2019-01-17')
SELECT
ReferenceNumber
,ReportData
,IIF(
LAG(ReportData) OVER(PARTITION BY ReferenceNumber ORDER BY ReportData)
= dateadd(day,-1,ReportData)
,'Not New'
,'New'
) AS NewAppt
FROM #TestData