Can someone explain how to write a SOQL query to get child custom object?

Question

I am a beginner. Can someone explain how to write a SOQL query to get child custom object when parent information is given and vice versa?
//student__c - child object, college__c-parent object, Related through Master detail

Answer 1

Query parent-to-child, which are almost always one-to-many. Specify these relationships using a subquery (enclosed in parentheses), where the initial member of the FROM clause in the subquery is related to the initial member of the outer query FROM clause. Note that for subqueries, you should specify the plural name of the object as that is the name of the relationship for each object.

SELECT Name, ( SELECT LastName FROM Contacts WHERE CreatedBy.Alias = 'x') FROM Account WHERE Industry = 'media'

Query child-to-parent relationships, which are often many-to-one. Specify these relationships directly in the SELECT, FROM, or WHERE clauses using the dot (.) operator.

SELECT Id, Name, Account.Name FROM Contact WHERE Account.Industry = 'media'