Question

Assume that I have 2 tables named aTable1, aTable2

aTable1 has userID set to identity and contains the following data:

userID  email          FirstName    LastName
1       NULL               C            CC
2       NULL               D            DD
3       [e-mail]        A            AA
4       [e-mail]        B            BB
5       [e-mail]        E            EE
6       [e-mail]        NULL         NULL
7       [e-mail]        NULL         NULL

aTable2 contains the following data:

userID  email          FirstName    LastName    Title
3       [e-mail]      A            AA       student
4       [e-mail]      B            BB       student
5       [e-mail]      E            EE       student
NULL    NULL             C            CC       dean
NULL    NULL             D            DD       advisor
NULL    [e-mail] NULL    NULL               student2
NULL    [e-mail] NULL    NULL               student3

I want to update aTable2.userID based on aTable1, but knowing that 2 tables have null values in it, so i do like this:

set ANSI_NULLS off
update aTable2
set aTable2.userID = a.userID
from aTable a, aTable2 b 
where a.FirstName = b.FirstName and a.LastName = b.LastName and a.email = b.email

However, what this update does not update all the userID, in fact, it only updates those that have email not equal to null, but I already set ANSI_NULLS to off.
What did I do wrong?

Answer 1

To get your update query to work, you can try something like this:

UPDATE a2
SET
   userId = a.UserId
FROM
    aTable2 a2
    JOIN aTable1 a ON 
        ISNULL(a.Email,'NULL') = ISNULL(a2.Email,'NULL') AND 
        ISNULL(a.FirstName,'NULL') = ISNULL(a2.FirstName,'NULL') AND
        ISNULL(a.LastName,'NULL') = ISNULL(a2.LastName,'NULL')

When the values are NULL, I’ve arbitrarily set the value to ‘NULL’ — use some distinct value that will not appear in your data to ensure you won’t receive false positives.

I’ve also seen other solutions using OR criteria in the JOIN and checking both values for NULL:

((a.Email = a2.Email) OR (a.Email IS NULL AND a2.Email IS NULL)) ...

Good luck.