sql - Count Distinct with NULL retention -

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i using sql 2005 , having simple query below trap duplicates:

select x,y,count(distinct z) z_count  tbla  group x,y  having (count(distinct z) > 1)

now issue column z having null values, ignored count distinct. such, duplicates having z null in 1 record , not-null in another, not getting trapped.

can plz suggest how can around 1 single query?

quick , dirty solution: replace null dummy-null value:

select x,y,count(distinct coalesce(z, 'dummy-null')) z_count  tbla  group x,y  having (count(distinct coalesce(z, 'dummy-null')) > 1)

only possible, if can fake dummy-null value.

edit: guess use count(distinct z) because rows having x=y=z shoud not considered duplicates (for reason). otherwise, duplicates of x=y use: 

select x,y,count(*) dup_count tbla  group x,y having (count(*) > 1)

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