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Oracle Tutorials - Filter Out Duplications in Returning Rows

By: FYIcenter.com

(Continued from previous topic...)

How To Filter Out Duplications in Returning Rows?

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

(Continued on next topic...)

  1. What Is a SELECT Query Statement?
  2. How To Select All Columns of All Rows from a Table?
  3. How To Select Some Columns from a Table?
  4. How To Select Some Rows from a Table?
  5. How To Sort the Query Output?
  6. Can the Query Output Be Sorted by Multiple Columns?
  7. How To Sort Query Output in Descending Order?
  8. How To Use SELECT Statement to Count the Number of Rows?
  9. Can SELECT Statements Be Used on Views?
  10. How To Filter Out Duplications in Returning Rows?
  11. What Are Group Functions?
  12. How To Use Group Functions in the SELECT Clause?
  13. Can Group Functions Be Mixed with Non-group Selection Fields?
  14. How To Divide Query Output into Groups?
  15. How To Apply Filtering Criteria at Group Level?
  16. How To Count Duplicated Values in a Column?
  17. Can Multiple Columns Be Used in GROUP BY?
  18. Can Group Functions Be Used in the ORDER BY Clause?
  19. How To Join Two Tables in a Single Query?
  20. How To Write a Query with an Inner Join?
  21. How To Define and Use Table Alias Names?
  22. How To Write a Query with a Left Outer Join?
  23. How To Write a Query with a Right Outer Join?
  24. How To Write a Query with a Full Outer Join?
  25. How To Write an Inner Join with the WHERE Clause?
  26. How To Write a Left Outer Join with the WHERE Clause?
  27. How To Name Query Output Columns?
  28. What Is a Subquery?
  29. How To Use Subqueries with the IN Operator?
  30. How To Use Subqueries with the EXISTS Operator?
  31. How To Use Subqueries in the FROM Clause?
  32. How To Count Groups Returned with the GROUP BY Clause?
  33. How To Return Top 5 Rows?

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