Here is how to emulate the EXISTS clause in SOQL
In the example below Education__c is a custom object that has a lookup field to Contact.
Suppose you have a query that would be written like this:
SELECT Id, Name
FROM Contact cn
WHERE EXISTS (SELECT 1
FROM Education__c
WHERE Contact__c = cn.Id )
It can be rewritten in SOQL using an Id Semi-Join as:
SELECT Id, Name
FROM Contact cn
WHERE Id IN (SELECT Contact__c
FROM Education__c )
It will retrieve all Contacts that have an associated Education__c record.
It is possible to use up to 2 Id Semi-Joins in a SOQL query (more information about it).