Let's say we want to display the names of students as well as authors, in a single column called "People".
In this case, we need to include data from two entirely different queries and present them in a single column:
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SELECT name FROM students
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SELECT name FROM authors
For such cases, SQL provides a combination clause called UNION.
Notice that the result contain 35 rows of names. There are 25 rows of students and 10 rows of authors.
Another key thing to notice about UNION is, the number of columns from each query should be equal.
Go ahead and run the query below. You will see an error.
We see an error because the first query in the UNION result produces two columns, while the second one produces only one, which is not acceptable in a UNION.