DISTINCT keyword in SQL is applied to a
SELECT statement to modify its behavior and return only unique records in the result.
For example, let’s say we wrote a query to return the first names from the
employees table. We would get a record returned for every row in the table, which would include duplicates if there were multiple employees with the same first name:
SELECT first_name FROM employees;
We can see this even more clearly if we also return the employee last names sort the results of the query alphabetically by first name using
SELECT first_name FROM employees ORDER BY first_name ASC;
Clearly, there are many employees with the same first name. If we wanted to return all the unique first names from the table, we can simply add the
DISTINCT clause to our original query. The
DISTINCT clause is placed before the columns to be returned in a
SELECT statement as below:
SELECT DISTINCT <columns> FROM <table>;
Therefore our original query becomes the below, and returns only the unique values in
first_name column in the
SELECT DISTINCT first_name FROM employees;
While our original query returned over 300K results (one first name for each employee), the above returns the distinct first names in the table of which there are only 1275.
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