MySQL 准备语句 - ? 在 WHERE 子句中

Question

This is a silly simple thing but I failed to google out an answer:

 PREPARE stmt1 FROM 'SELECT productCode, productName

                 FROM products

                 WHERE productCode = ?';

What is the ? at the end? '?' is not a wildcard in SQL so what is it...

-- Edit: thank you!

Answer 1

? is called a parameter placeholder.

The statement may not be complete, because data values that are unknown at preparation time are represented by the question mark characters that serve as parameter markers. At the time the statement is executed, you provide specific data values, one for each parameter in the statement.

• MySQL - Prepared Statements

Answer 2

Are you using an ORM such as Doctrine? ORMs usually put placeholders (?) in place of values and only apply them before execution.

Answer 3

? is used as a placeholder for the values that are not known in advance.

Suppose that you have two ? in your statement, then you can define what parameter what values will be in place of them by assigning them the values using the index of ? (as we do in java)

I am using an example of Java to make you understand PreparedStatement :

Let say my query is :

PreparedStatement ps = connection.prepareStatement(
                    `select * from employee where ename = ? and location = ?`);

So I have defined the placeholder for the values to be checked against ename and location while executing the query.

Now I set the values without changing my query (as we in java)

ps.setString(1,"John");
ps.setString(2,"New York");

So now the query at run time will be :

select * from employee where ename = 'John' and location = 'New York'

And next time if I want to change the values then I will do :

ps.setString(1,"Williams");
ps.setString(2,"London");

So now my query will be :

select * from employee where ename = 'Williams' and location = 'London'