SQL Server/Oracle/MySQL Union query

Many time we require results from different tables or different queries. We try one after the other and list the result accordingly. There is a better way to do this by using UNION command or query. We can list records of two different tables by combining two sql queries to one.
What is the advantage ?

We can as well write the query two times and display one after the other. The point here is we can pick up unique records also. Say we have two tables one is storing name of the football players and other is storing players of baseball team. We want to display a list of players who are member of one of the team at least. Here we can display the name of the players by using UNION command. If John is member of football and baseball team then his name will appear once only. If we want to display all the records then we can use ALL along with the UNION command.

Here are two tables with three records each. Please note that there is one common record . Name Alex is there in both the tables.
Baseball Football
name age
Reid 16
Alex 21
Greek 19
name age
John 20
Alex 21
Robort 23


SELECT name,age FROM football
UNION
SELECT name,age FROM baseball

OUT put is >
name age
John 20
Alex 21
Robort 23
Reid 16
Greek 19
You can see from the above output that only unique records are displayed so the record Name Alex is displayed once. If the requirement is to display all the records then we have to add ALL command to the UNION query

SELECT name,age FROM football
UNION ALL
SELECT name,age FROM baseball

OUT put is >
name age
John 20
Alex 21
Robort 23
Reid 16
Alex 21
Greek 19

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