In Oracle, exists clause is used with select, insert, update, delete statements. It is used to combine the queries and creating subquery.
Where exists (subquery)
subquery: It is a select statement which returns at least one record set.
Query: select name from table1 where exists (select *from table2 where table1.id=table2.id)
Query: select id, salary from table2 where exists (select *from table1 where table2.id=table1.id)
Query: select * from table1 where not exists (select *from table2 where table1.id=table2.id)