Insert value only when column is null - sql

Question

I have 2 different Excel sources
Employee Excel source
Sales Excel source
In my employee source I have multiple folder and in my sales source I have multiple folders as well as multiple sheets, so I had to use two data flow task and two for each loop containers for that purpose.
I need to take the ID column from my employee Excel source, if the ID column in my employee source is not available (null) only if not available in my employee source then I need to get the ID Column from my sales source, they have matching SSN, how can I achieve this?
This is my staging table
SELECT
ID, [SSN],
[Borrower FirstName], [Borrower LastName],
[Borrower Email], [Home Phone], [Cell Phone],
[Marital Status], [Date of Birth],
[Current Street Address], [City], [State], [Zip],
[YearsAtThisAddress], [Sex], [Ethnicity], [Race]
FROM
[Mortgage]
This is my staging table, half of my data from the employee and half from sales.
Employee source has
[ID] ,[SSN] ,[Borrower FirstName] ,[Borrower LastName] ,[Borrower Email] ,[Home Phone] ,[Cell Phone] ,[Marital Status] ,[Date of Birth] ,[YearsAtThisAddress]
and SALES source has
[ID] ,[SSN] ,[Borrower FirstName] ,[Borrower LastName] ,[Borrower Email] ,[Home Phone] ,[Cell Phone] ,[Current Street Address] ,[City] ,[State] ,[Zip]

Answer 1

You have records from two data sources merged together and part of your requirement is to know from which source the record came (whether the ID column is NULL only if from the employee source). Our only clue as to whether a record came from the "employee source" is if there are values in the columns that differ between the two ([Marital Status], [Date of Birth], [YearsAtThisAddress]), but you don't say if any of these nullable, so we must make an assumption. Assuming the value in column [YearsAtThisAddress] can be used to determine the record's source (NOT NULL means "Employee" while NULL means "Sales"), you could do something like this to return the corresponding "Sales" ID number for records matching on SSN when the "Employee" ID number is NULL.
SELECT
CASE WHEN ID IS NULL AND [YearsAtThisAddress] IS NOT NULL
THEN (SELECT TOP (1) ID
FROM [Mortgage]
WHERE [YearsAtThisAddress] IS NULL
AND ID IS NOT NULL
AND [SSN] = m.[SSN]
ORDER BY ID)
ELSE ID
END AS ID,
[SSN],
[Borrower FirstName], [Borrower LastName],
[Borrower Email], [Home Phone], [Cell Phone],
[Marital Status], [Date of Birth],
[Current Street Address], [City], [State], [Zip],
[YearsAtThisAddress], [Sex], [Ethnicity], [Race]
FROM
[Mortgage] AS m