In a SOQL statement, what is the =: symbol?

Question

How does = differ from =: ?

My code snippet below only works when using =:, however when looking through the SOQL documentation, I mostly see examples using =.

public class AccountUtils {

    public static List<Account> accountsByState(String abbr) {
        list<Account> result = [SELECT ID, Name FROM Account WHERE BillingState =: abbr];
        system.debug(result);
        return result; 
    }
}

Versus an example from the documentation that uses =:

SELECT Id
FROM Case
WHERE Contact.LastName = null

Answer 1

The : denotes a variable, in your case for abbr in your SOQL e.g.

SELECT ID, Name FROM Account WHERE BillingState = :abbr

as opposed to null which is a literal value

SELECT Id FROM Case WHERE Contact.LastName = null

in short if you are using a variable in SOQL then use : before the variable name.

Answer 2

SOQL and SOSL statements in Apex can reference Apex code variables and expressions if they’re preceded by a colon (:). This use of a local code variable within a SOQL or SOSL statement is called a bind. The Apex parser first evaluates the local variable in code context before executing the SOQL or SOSL statement. Bind expressions can be used as:

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_SOQL_variables.htm#!