Let's say we want to display all the ids common to students and books.
In this case, we need to include data from two entirely different queries:
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SELECT id FROM students
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SELECT id FROM books
and get common ids from them together in a single column.
For such a use case, SQL provides a clause similar to UNION, called INTERSECT, which helps display common data of different types together, from results of two or more queries.
Notice that we get only ids 1 to 25 in the results. students have ids ranging from 1 to 25, books have ids ranging from 1 to 34. The common data amongst the two results is 1 to 25, hence the result from the INTERSECT query above.