Salesforce Cheat Sheets 

https://developer.salesforce.com/page/Cheat_Sheets

what is the difference between != null and != ' '

!= null checks if the value is equal to null 

!= ' '  will check for a blank

id field can be either hold a 15 or 18 digit id or can be null but not blank

for string use string.isblank() returns true if string is space ,empty or null

Is there a way to Print the Number of SOQL Queries in Debug During Runtime?

System.debug('Total Number of SOQL Queries allowed in this apex code context: ' +  Limits.getLimitQueries());

System.debug('Total Number of SOQL Queries fired in this apex code context: ' +  Limits.getQueries());