Check this out: Using Apex Variables in SOQL and SOSL Queries | Apex Developer Guide | Salesforce Developers

Account A = new Account(Name='xxx');
insert A;
Account B;

// A simple bind
B = [SELECT Id FROM Account WHERE Id = :A.Id];

// A bind with arithmetic
B = [SELECT Id FROM Account 
     WHERE Name = :('x' + 'xx')];

String s = 'XXX';

// A bind with expressions
B = [SELECT Id FROM Account 
     WHERE Name = :'XXXX'.substring(0,3)];

// A bind with INCLUDES clause
B = [SELECT Id FROM Account WHERE :A.TYPE INCLUDES (‘Customer – Direct; Customer – Channel’)];

// A bind with an expression that is itself a query result
B = [SELECT Id FROM Account
     WHERE Name = :[SELECT Name FROM Account
                    WHERE Id = :A.Id].Name];

Contact C = new Contact(LastName='xxx', AccountId=A.Id);
insert new Contact[]{C, new Contact(LastName='yyy', 
                                    accountId=A.id)};

// Binds in both the parent and aggregate queries
B = [SELECT Id, (SELECT Id FROM Contacts
                 WHERE Id = :C.Id)
     FROM Account
     WHERE Id = :A.Id];

// One contact returned
Contact D = B.Contacts;

// A limit bind
Integer i = 1;
B = [SELECT Id FROM Account LIMIT :i];

// An OFFSET bind
Integer offsetVal = 10;
List<Account> offsetList = [SELECT Id FROM Account OFFSET :offsetVal];

// An IN-bind with an Id list. Note that a list of sObjects
// can also be used--the Ids of the objects are used for 
// the bind
Contact[] cc = [SELECT Id FROM Contact LIMIT 2];
Task[] tt = [SELECT Id FROM Task WHERE WhoId IN :cc];

// An IN-bind with a String list
String[] ss = new String[]{'a', 'b'};
Account[] aa = [SELECT Id FROM Account 
                WHERE AccountNumber IN :ss];

// A SOSL query with binds in all possible clauses

String myString1 = 'aaa';
String myString2 = 'bbb';
Integer myInt3 = 11;
String myString4 = 'ccc';
Integer myInt5 = 22;

List<List<SObject>> searchList = [FIND :myString1 IN ALL FIELDS 
                                  RETURNING 
                                     Account (Id, Name WHERE Name LIKE :myString2
                                              LIMIT :myInt3), 
                                     Contact, 
                                     Opportunity, 
                                     Lead 
                                  WITH DIVISION =:myString4 
                                  LIMIT :myInt5];