The DISTINCT operator in a SELECT statement filters the result set to remove duplicates.

-- Returns the unique values from one column.
-- NULL is included in the set of values if any rows have a NULL in this column.
SELECT DISTINCT c_birth_country FROM customer;

-- Returns the unique combinations of values from multiple columns.
SELECT DISTINCT c_salutation, c_last_name FROM customer;

You can use DISTINCT in combination with an aggregation function, typically COUNT(), to find how many different values a column contains.

-- Counts the unique values from one column.
-- NULL is not included as a distinct value in the count.
SELECT COUNT(DISTINCT c_birth_country) FROM customer;

-- Counts the unique combinations of values from multiple columns.
SELECT COUNT(DISTINCT c_salutation, c_last_name) FROM customer;

Zero-length strings: For purposes of clauses such as DISTINCT and GROUP BY, Impala considers zero-length strings (""), NULL, and space to all be different values.