How to avoid duplicate data in SQL?

When retrieving data from a table, there might be some data rows with the same value of a column.

To avoid such duplication, we can add DISTINCT to the SELECT statement:

SELECT DISTINCT column_name 
FROM table_name
WHERE [condition]

Example

We have this table upcoming_movies:

movie_name	release_year	universe
Joker	2019	DC
The New Mutants	2020	Marvel
Black Widow	2020	Marvel
Birds of Prey	2020	DC
The Eternals	2020	Marvel
Wonder Woman 1984	2020	DC
Doctor Strange and the Multiverse of Madness	2021	Marvel
Thor: Love and Thunder	2021	Marvel
The Batman	2021	DC
The Suicide Squad	2021	DC
Black Panther 2	2022	Marvel
Aquaman 2	2022	DC

To SELECT all universe value without duplication:

SELECT DISTINCT
  universe
FROM
  upcoming_movies;

The result is:

universe
DC
Marvel

When SELECT DISTINCT clause contains more than one column, the values of all the columns combined determine the uniqueness of rows.

For example, with the upcoming_movies above, we SELECT DISTINCT two columns universe and release_year:

SELECT DISTINCT
  universe,
  release_year
FROM
  upcoming_movies;

The results will be:

universe	release_year
DC	2019
Marvel	2020
DC	2020
Marvel	2021
DC	2021
Marvel	2022
DC	2022

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