The optional DISTINCT keyword eliminates duplicate rows from the results of a SELECT statement.
If you do not specify DISTINCT, you get all rows, including duplicates. Optionally, you can specify ALL before the select list to get all rows. For compatibility with other implementations of SQL, SQL Anywhere syntax allows the use of ALL to explicitly ask for all rows. ALL is the default.
For example, if you search for all the cities in the Contacts table without DISTINCT, you get 60 rows:
SELECT City FROM Contacts;
You can eliminate the duplicate entries using DISTINCT. The following query returns only 16 rows.:
SELECT DISTINCT City FROM Contacts;
The DISTINCT keyword treats NULL values as duplicates of each other. In other words, when DISTINCT is included in a SELECT statement, only one NULL is returned in the results, no matter how many NULL values are encountered.