Final answer:
A Semi-Join in SOQL is used to filter records from one object based on related records in another object, with the query structure resembling a nested SELECT statement. It allows for retrieving records without returning the related records themselves.
Step-by-step explanation:
A Semi-Join in SOQL (Salesforce Object Query Language) is a way to query records from one object that are related to records in another object, without actually returning the related records themselves. It is akin to a nested SELECT statement where the inner query's results are used to filter the records returned by the outer query.
For example, if you want to retrieve all accounts that have at least one related contact, but you only want the account information, you would use a semi-join. The query would look something like:
SELECT Id, Name FROM Account WHERE Id IN (SELECT AccountId FROM Contact)
This query selects only the IDs and Names of the accounts that are related to contacts, effectively filtering out any account records that do not have associated contact records.