what is use of question mark in sql [duplicate]

Question

I was just surfing the net and found a query something like:

sql  = "select milk_rate from special_milk_rate 
        where code_producer_id=? and effective_from <= ? 
        and effective_till >= ?"

what exactly this query means i means what is the use of ? in this statement.

and one thing more what is use of & in sql.

Answer 1

This usually implies a prepared statement, where the parameters are filled in later. (see e.g. http://en.wikipedia.org/wiki/Prepared_statements#Parameterized_statements).

Answer 2

what exactly this query means i means what is the use of ? in this statement.

The question marks are for parameters.

and one thing more what is use of & in sql.

& is a bitwise AND operator in sql