T-SQL查询：不在列中的列给出了错误的结果

Question

I have two table A and table B, the common column in both table is Name, I want to know what are name in Table A that is not in table B

when I do:

Select Name from A where Name not in (Select Name from B)

I am sure there are 2 name in Table A that is not in table B

but the result returns nothing

These name column in table A and B has the same datatype varchar(50)

so I copy the result of Name column and Insert into a new table and do the same query, and this time it returns the right result. what bug could this be?

example:

Table A
Name:
Kevin
Dexter
David 
John
Marry

Table B
Name:
Kevin
Dexter
David 

So the query should return 'John' , 'Marry' but it doesn't return in my original table, but it returns in another table I create and insert.

Thanks!

Answer 1

You probably have a NULL name on B , this makes the NOT IN false for every row. You should use NOT EXISTS instead:

SELECT Name 
FROM A 
WHERE NOT EXISTS (SELECT 1 FROM B 
                  WHERE A.Name = B.Name)

Answer 2

Obviously it is because of NULL value on some rows in table B. You can do what you want with EXCEPT

SELECT Name FROM TableA
EXCEPT
SELECT Name FROM TableB

Answer 3

SELECT a.Name
FROM TableA a
LEFT JOIN TableB b ON b.Name = a.Name
WHERE b.Name IS NULL