Question

I have a database that contains 5 digit zip codes (ie 10001) and matching state abbreviations (ie NJ, NY, CA). I’ve found that some of the zip codes have multiple states (ie 10001 = NJ and 10001 = NY) which is wrong.

   zip         State
 10001          NY
 10001          NJ
 10001          NY
 10001          NY
  ...           ...

Each State can have many zip codes, but each zip code should have only one state.

I’d like to find all the errors but can’t seem to write a query to do so.

Any suggestions?

Answer 1

A different approach, but rather than just give you a count, this gives you the states involved.

SELECT zip, state 
FROM dbo.table AS t
WHERE EXISTS 
(
  SELECT 1 FROM dbo.table 
  WHERE zip = t.zip AND state <> t.state
)
GROUP BY zip, state
ORDER BY zip, state;

Once you’ve identified the duplicates and removed them, add a unique constraint on zip,state so you’re not doing this again next week, next month, etc.