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An Introductory Guide to SQL

Union

Explore how to combine results from different SQL queries using the UNION clause. Understand its syntax, requirements for column matching, handling duplicates, and the difference between UNION and UNION ALL. Practice writing queries that merge data sets while maintaining data integrity and compatibility.

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Union

The UNION clause allows us to combine the results from several queries together. The clause doesn’t join the table but merely clubs the two results together.

Syntax

<Query1>

UNION

<Query2>

Connect to the terminal below by clicking in the widget. Once connected, the command line prompt will show up. Enter or copy and paste the command ./DataJek/Lessons/27lesson.sh and wait for the MySQL prompt to start-up.

MySQL
-- The lesson queries are reproduced below for convenient copy/paste into the terminal. 



-- Query 1

SELECT FirstName FROM Actors 

UNION 

SELECT URL FROM DigitalAssets;



-- Query 2

(SELECT CONCAT(FirstName, ' ', SecondName) AS "Actor Name" 

FROM Actors 

ORDER BY NetworthInMillions DESC 

LIMIT 2)

UNION

(SELECT CONCAT(FirstName, ' ', SecondName) AS "ThisAliasIsIgnored" 

FROM Actors 

ORDER BY NetworthInMillions ASC 

LIMIT 2);



-- Query 3

SELECT FirstName, Id FROM Actors 

UNION 

SELECT FirstName FROM Actors;



-- Query 4

SELECT FirstName, Id FROM Actors 

UNION 

SELECT FirstName, null FROM Actors;



-- Query 5

SELECT MaritalStatus FROM Actors 

UNION 

SELECT Gender FROM Actors;



-- Query 6

SELECT MaritalStatus FROM Actors 

UNION ALL

SELECT Gender FROM Actors;



-- Query 7

(SELECT CONCAT(FirstName, ' ', SecondName) AS "Actor Name"  

FROM Actors  

ORDER BY NetworthInMillions DESC  LIMIT 2)  

UNION  

(SELECT NetworthInMillions 

FROM Actors 

ORDER BY NetworthInMillions ASC);



-- Query 8

(SELECT CONCAT(FirstName, ' ', SecondName) AS "Actor Name"  

FROM Actors  

ORDER BY NetworthInMillions DESC  LIMIT 2)  

UNION  

(SELECT NetworthInMillions 

FROM Actors 

ORDER BY NetworthInMillions ASC LIMIT 3);
Terminal 1
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  1. As a contrived example, we’ll write a query that prints all the first names from the Actors table and all the URLs from the DigitalAssets table.

    SELECT FirstName FROM
...