Getting table(records) to update properply using the MERGE Statement - sql

Question

Good morning everyone!
Below is a piece of code I stitched together: I used a CTE to grab the records(data) from a link table and than convert strings to dates, than use the merge statement to get the data into a local table:
I am having a problem with the column(field) LAST_RACE_DATE this field is set to NULL and is not required but it does not update with my current set up. What I am trying to accomplished is for this field to populate when data is entered but also update, meaning it should also update with NULL.
So if the field has a specific date, and a new date is entered in the remote database, this field should update as well, even if the data is deleted in the back end, it should also remove the local table data for this field.
WITH CTE AS(
SELECT MEMBER_ID
,[MEMBER_DATE] = MAX(CONVERT(DATE, MEMBER_DATE))
,RACE_DATE = MAX(CONVERT(DATE, RACE_DATE))
,LAST_RACE_DATE = MAX(CONVERT(DATE, LAST_RACE_DATE))
FROM [EXAMPLE].[dbo].[LINKED_MEMBER_DATA]
WHERE (MEMBER_DATE IS NOT NULL) AND (ISDATE(MEMBER_DATE)<> 0) AND (RACE_DATE IS NOT NULL) AND (ISDATE(RACE_DATE)<> 0)
AND (LAST_RACE_DATE IS NULL) OR (ISDATE(LAST_RACE_DATE)<> 0)
GROUP BY MEMBER_ID)
MERGE dbo.LINKED_MEMBER_DATA AS Target
USING (SELECT
MEMBER_ID, MEMBER_DATE, RACE_DATE, LAST_RACE_DATE
FROM CTE
GROUP BY MEMBER_ID, RACE_DATE, LAST_RACE_DATE)AS SOURCE ON (Target.MEMBER_ID = SOURCE.MEMBER_ID)
WHEN MATCHED AND
(Target.MEMBER_DATE) <> (SOURCE.MEMBER_DATE)
OR (Target.RACE_DATE) <> (SOURCE.RACE_DATE)
OR ISNULL(TARGET.LAST_RACE_DATE , Target.LAST_RACE_DATE) <> ISNULL(SOURCE.LAST_RACE_DATE, SOURCE.LAST_RACE_DATE)
THEN UPDATE SET
Target.MEMBER_DATE = SOURCE.MEMBER_DATE
,Target.RACE_DATE = SOURCE.RACE_DATE
,Target.LAST_RACE_DATE = SOURCE.LAST_RACE_DATE
WHEN NOT MATCHED BY TARGET THEN
INSERT(
MEMBER_ID, MEMBER_DATE, RACE_DATE, LAST_RACE_DATE)
VALUES (Source.MEMBER_ID, Source.MEMBER_DATE, Source.RACE_DATE, Source.LAST_RACE_DATE);
I also tried this:
ISNULL(Target.LAST_RACE_DATE,'N/A') <> ISNULL(SOURCE.LAST_RACE_DATE,'N/A')
But it generates the below error for dates conversion:
Conversion failed when converting date and/or time from character string.
Thanks a Million!!

Answer 1

Your current statement is failing because the ISNULLs that you have don't do anything (if one of the values is NULL the expression will evaluate to NULL), and NULL values don't compare. Your second attempt doesn't work because ISNULL requires the data types of the two values to be the same, so you could try eg ISNULL(Target.LAST_RACE_DATE, '1970-01-01') <> ISNULL(Source.LAST_RACE_DATE, '1970-01-01').
Another option would be to simply enumerate the different cases (eg, (((Source.LAST_RACE_DATE IS NULL AND Target.LAST_RACE_DATE IS NOT NULL) OR (Source.LAST_RACE_DATE IS NOT NULL AND Target.LAST_RACE_DATE IS NULL) OR (Source.LAST_RACE_DATE <> Target.LAST_RACE_DATE))). Enumerating the different situations makes the code a bit more verbose, but it can result in better performance (whether it is measurably better really depends on how much data you are processing).