Finding duplicates using SQL - sql

Question

I am trying to write a query that will display all duplicates in a table.
I have a table, lets call it WORKERS. This table has multiple columns; the two I am focusing on are called SocialSecurityNbr and EmpNbr.
I would like the query to display all rows where
SocialSecNbr == SocialSecNbr
AND
EmpNbr != EmpNbr
Below I have an example of my data followed by what I want the output to show. (for the simplicity of this question I have only used 4 digits to represent the social security number)
ID EmpNbr SocialSecNbr EmpName
1 00001 9711 Smith,John
2 00002 5789 Harris, Greg
3 00001 9711 Smith,John
4 00003 4100 Thompson,Lisa
5 00004 1250 Fulton,Kyle
6 00005 3999 Harris, Amber
7 00004 1250 Fulton,Kyle
8 00007 1250 Morlan,Richard
9 00008 3999 Levy,Harold
What I would like to see as the output:
ID EmpNbr SocialSecurityNbr EmpName
5 00004 1250 Fulton,Kyle
6 00005 3999 Harris, Amber
7 00004 1250 Fulton,Kyle
8 00007 1250 Morlan,Richard
9 00008 3999 Levy,Harold
As you can see above all of the duplicate Social Security numbers are shown in the output, except for John Smith. In the actual table there are many instances where the same person is shown more than once, this is fine and I do not what to see this in the outcome.
I have searched online for information on how to do this but all I found was examples using "Count > 1". I'm thinking I need to use "Distinct" however I do not believe that I can apply that function to just one column.

Answer 1

At least for ms-sql joining table with itself will work:
select distinct w1.ID, w1.EmpNbr, w1.SocialSecNbr, w1.EmpName
from WORKERS w1
inner join WORKERS s2 on w1.SocialSecNbr = s2.SocialSecNbr
AND
w1.EmpNbr <> s2.EmpNbr
for other sql flavours it should work as well.
See sample at SqlFiddle

Answer 2

You can approach this using an exists clause:
select ID, EmpNbr, SocialSecurityNbr, EmpName
from workers w
where exists (select 1
from workers w2
where w2.SocialSecurityNbr = w.SocialSecurityNbr and
w2.EmpNbr <> w.EmpNbr
);
With an index on workers(SocialSecurityNbr, EmpNbr), this should be relatively efficient.

Answer 3

The query below will show you all rows in Workers where the SocSecurityNbr has "duplicates" (as defined by having multiple different EmpNames).
SELECT *
FROM Workers
WHERE SocSecurityNbr IN (
SELECT SocSecurityNbr
FROM Workers
GROUP BY SocSecurityNbr
HAVING COUNT(DISTINCT EmpName) > 1
)
You could easily modify this to change your definition of a "duplicate" - for example, if there are multiple different employee numbers.