What is wrong with SOQL Condition Expression Syntax (WHERE Clause)

Question

1) Working:

SELECT CreatedDate, Account.CreatedDate FROM Opportunity
WHERE CALENDAR_YEAR(CreatedDate) = 2016

2) Working:

SELECT CreatedDate, Account.CreatedDate FROM Opportunity
WHERE CALENDAR_YEAR(Account.CreatedDate) = 2016

Above two SOQL are working in developer console. But SOQL below is not working. I want to know why SOQL WHERE clause behave like this OR there are any syntax error in my SOQL

3) Unknown error parsing query

SELECT CreatedDate, Account.CreatedDate FROM Opportunity
WHERE CALENDAR_YEAR(CreatedDate) = CALENDAR_YEAR(Account.CreatedDate)

4) Unknown error parsing query

SELECT CreatedDate, Account.CreatedDate FROM Opportunity
WHERE CALENDAR_YEAR(CreatedDate) = : CALENDAR_YEAR(Account.CreatedDate)

Answer 1

Each element of the where clause is limited to being only a fieldExpression:

fieldName comparisonOperator value

where the value cannot be a reference to another field.