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 ggl_team AS

SELECT first_name, last_name FROM employees

WHERE first_name = 'John';

Table created.

SQL> INSERT INTO ggl_team VALUES ('John', 'Chen');

SQL> INSERT INTO ggl_team VALUES ('James', 'Chen');

SQL> INSERT INTO ggl_team VALUES ('Peter', 'Chen');

SQL> INSERT INTO ggl_team VALUES ('John', 'Chen');

SQL> SELECT * FROM ggl_team;

FIRST_NAME   LAST_NAME

-------------------- -------------------------

John    Chen

John    Russell

John    Seo

John    Chen

James   Chen

Peter   Chen

John    Chen

SQL> SELECT DISTINCT * FROM ggl_team;

FIRST_NAME   LAST_NAME

-------------------- -------------------------

Peter   Chen

John    Chen

James   Chen

John    Seo

John    Russell