Question

I’ve seen some (my)sql queries written like this:

SELECT ? + 4;

What’s the meaning of ? ? My guess is it’s some kind of parameter, but how does one specify the value for it?

Answer 1

? is a placeholder for parameter values in the syntax for prepared statements. The linked article gives the following example:

mysql> PREPARE stmt1 FROM 'SELECT SQRT(POW(?,2) + POW(?,2)) AS hypotenuse';
mysql> SET @a = 3;
mysql> SET @b = 4;
mysql> EXECUTE stmt1 USING @a, @b;
+------------+
| hypotenuse |
+------------+
|          5 |
+------------+
mysql> DEALLOCATE PREPARE stmt1;

Since you also tagged sql, it’s worth linking to the Prepared Statement Wikipedia article for further reading independent of MySQL.