Vanilla SQL that selects multiple values in single column

Question

If I have a table with customer IDs in one column and time zones in another, is there a plain SQL statement that can select all customer IDs that have different time zone values? In other words, I want to find those customers with offices in New York, Chicago, and San Francisco but not those who ONLY have an office in one or the other time zones.

Answer 1

SELECT Customer
FROM MyTable
GROUP BY Customer
HAVING COUNT(DISTINCT TimeZone) > 1

The use of DISTINCT is important.

COUNT(TimeZone) counts all non-null values, not just distinct values. So it's equivalent to COUNT(*) except where TimeZone is null.

In other words, if a given customer has three offices, but all are in the Eastern timezone, COUNT(TimeZone) will be 3, whereas COUNT(DISTINCT TimeZone) will be 1.

Answer 2

SELECT Customer
FROM MyTable
GROUP BY Customer
HAVING COUNT(DISTINCT TimeZone) > 1

Answer 3

ugly, but effective:

select CustomerID
where  CustomerID in 
(
select customerID from
     (select distinct CustomerID
     from   table
     where  TimeZone in ('NY','CHI','SF')) t
having count(*) = 3
)