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.