Subqueries - EXISTS operator
Let’s say we want to show
names of students who are enrolled in courses that have at least 1 book that belong to them.
This problem asks for selecting records from a table on the basis that whether there is at least one record existing with a certain condition related to the original table.
SQL provides an operator called
EXISTS with subqueries, that serves the exact same use case. Have a look at the query below.
Observe, that the query results does not include students, who do not have any course information. They also do not include students who are enrolled in courses such as
SQL, which have no books that belong to them.
Notice that the
EXISTS operator here works as a boolean filter. If there is at least one record that gets returned from the subquery, the operator returns
true, otherwise it returns