Does NULL count as a unique value?

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Accepted Answer

COUNT(DISTINCT column) counts unique non-NULL values. The distinct customers are Alice and Bob - that is 2. NULL is excluded because COUNT(column) always ignores NULLs, whether or not DISTINCT is used. Compare: COUNT(*) = 4 (all rows), COUNT(customer) = 3 (non-NULL), COUNT(DISTINCT customer) = 2 (unique non-NULL).