How To Filter Out Duplications in the Returning Rows? - Oracle DBA FAQ - Understanding SQL SELECT Query Statements

If there are duplications in the returning rows, and you want to remove the duplications, you can use the keyword DISTINCT or UNIQUE in the SELECT clause. The tutorial exercise below shows you that DISTINCT works on selected columns only:

SQL> CREATE TABLE fyi_team AS 
  SELECT first_name, last_name FROM employees 
  WHERE first_name = 'John';
Table created.

SQL> INSERT INTO fyi_team VALUES ('John', 'Chen');
SQL> INSERT INTO fyi_team VALUES ('James', 'Chen');
SQL> INSERT INTO fyi_team VALUES ('Peter', 'Chen');
SQL> INSERT INTO fyi_team VALUES ('John', 'Chen');

SQL> SELECT * FROM fyi_team;
FIRST_NAME           LAST_NAME
-------------------- -------------------------
John                 Chen
John                 Russell
John                 Seo
John                 Chen
James                Chen
Peter                Chen
John                 Chen

SQL> SELECT DISTINCT * FROM fyi_team;
FIRST_NAME           LAST_NAME
-------------------- -------------------------
Peter                Chen
John                 Chen
James                Chen
John                 Seo
John                 Russell

SQL> SELECT DISTINCT last_name FROM fyi_team;
LAST_NAME
-------------------------
Chen
Russell
Seo