SQL: IS NULL fails when trying to update values - sql

Question

I am currently trying to update some table values and I am stuck with a particular instance.
The situation is that I have a main table DBO.MAIN_INTERACTIONS that I join with an external table. Based on the below WHERE values I would then like to update the main database values. KCMACustomer.DBO.DATA_EXT_GREEN_ENR is a table that is linked to one particular product and my idea was to select and update the table rows of MAIN that do not have a connection with this GREEN_ENR table. That's why I put the EXT.NUMBER_OF_ACCOUNTS IS NULL.
So the rows I am trying to retrieve will have no match on this JOIN condition 'MAIN.GENID = EXT.GENID'. Is this what makes my update query fail? and would there be a better way to get the rows that have no connection with KCMACustomer.DBO.DATA_EXT_GREEN_ENR (the main table values are all the same, so I can't differentiate there)
Extra info: the second query does work, probably because there is a succesful 'MAIN.GENID = EXT.GENID' join
UPDATE
MAIN
SET
STATE_CODE='S8',
STATE_NAME='Reminder 1',
OLD_STATE='S4',
MODIFIED_DT = #NOW
OUTPUT INSERTED.ID, 0, 'AUTO-STATE','business','Doc.expected -> Reminder 1',INSERTED.CAMPAIGNID, #NOW,'S4','S8' INTO KCMACustomer.DBO.DATA_EBW_FFC_LOG_STATES
FROM KCMACUSTOMER.DBO.MAIN_INTERACTIONS AS MAIN
JOIN KCMACustomer.DBO.DATA_EXT_GREEN_ENR AS EXT
ON MAIN.GENID = EXT.GENID
WHERE
MAIN.STATE_CODE='S4'
AND
MAIN.TYPE_DEMAND='S4'
AND
EXT.NUMBER_OF_ACCOUNTS IS NULL
AND
DATEDIFF(hh, MAIN.MODIFIED_DT, #NOW)>=168
AND
MAIN.PRODUCT IN (#HELLO4YOU, #COMFORT_PACK, #PREMIUM_PACK)
In this case the 'MAIN.GENID = EXT.GENID' will have a match and it does update the records I want
UPDATE
MAIN
SET
STATE_CODE='S8',
STATE_NAME='Reminder 1 eID',
OLD_STATE='S4',
MODIFIED_DT = #NOW
OUTPUT INSERTED.ID, 0, 'AUTO-STATE','business','Doc expected -> Reminder 1 eID',INSERTED.CAMPAIGNID, #NOW,'S4','S8' INTO KCMACustomer.DBO.DATA_EBW_FFC_LOG_STATES
FROM KCMACUSTOMER.DBO.MAIN_INTERACTIONS AS MAIN
JOIN KCMACustomer.DBO.DATA_EXT_GREEN_ENR AS EXT
ON MAIN.GENID = EXT.GENID
WHERE
MAIN.STATE_CODE = 'S4'
AND
DATEDIFF(hh, MAIN.MODIFIED_DT, #NOW)>=120
AND
MAIN.PRODUCT IN (#HELLO4YOU, #COMFORT_PACK, #PREMIUM_PACK)
AND
EXT.NUMBER_OF_ACCOUNTS IS NOT NULL
AND
MAIN.DEMAND_DT > '2020-05-27 00:00:00'

Answer 1

JOIN implies an INNER JOIN
The use case you described requires a LEFT JOIN.
You will need to change your query so that it uses
LEFT JOIN KCMACustomer.DBO.DATA_EXT_GREEN_ENR AS EXT
ON MAIN.GENID = EXT.GENID
I also suggest to test the NULL condition of the same column you use in the join condition. So instead of doing
EXT.NUMBER_OF_ACCOUNTS IS NULL
check if
EXT.GENID IS NULL
You are more familiar with your data so it might not have an impact on your query. But the record from MAIN could be linked to a record in EXT, but that record could have NUMBER_OF_ACCOUNTS NULL.
However, checking on the GENID of the EXT table would ensure that a link was not found when trying to find a matching record in main.