Handle decimal numbers in mysqli

Question

I have to put the price of some items inside a mysql table. When creating the table I'm using DECIMAL(10,2) as I don't need more than 2 digits after the comma (for example: 123,45 would be accepted as an input but 123,456 would be rounded to 123,45 with PHP).

First question: using DECIMAL(10,2), how do I know how many numbers may be stored before the comma? I know it is not 10, as 10 is just the precision Mysql uses when doing math with those numbers: so where the length of the number itself is specified?

Second question: I'm using PHP to round user input to fit the data type (float with 2 numbers after the comma). How should I use mysqli->bind_param to insert those data? Which of these datatypes (from the documentation) accepted by bind_param should I use (and possibly: why)?

Character   Description
i   corresponding variable has type integer
d   corresponding variable has type double
s   corresponding variable has type string
b   corresponding variable is a blob and will be sent in packets

Solution

DECIMAL(10,2) means that 10 is the maximum number of digits to be used. The 2 specifies that there are two on the right side of the decimal. So this means there are 8 digits left on the left side. See the manual

As for the datatype, you should use double as this is for decimal numbers.

Integer has no decimal chars, so it will floor all numbers. String is for text and not numeric values. Blob is for a binary large object, EG: an image

Comments

[Bill Karwin]

I don't recommend using double or float if you need exact scaled numerics. See dev.mysql.com/doc/refman/5.6/en/problems-with-float.html

[Hugo Delsing]

In this question and answer mysql uses decimal. its only the bind param type that is set to double. And itsthe only option from the types given.

[Dane Caswell]

You could just use 's' String. Provided the data type you are using in php to store the value (Decimal) has a __toString method (if not create one) then the PHP with convert the value to string before its passed to the MySQL engine and the engine will perform its own type casts to process the appropriate result for the field type. Can be useful for types like decimal that require more precision than the integer or even double can provide.

[Hugo Delsing]

Sure, it would work, but the whole point of binding types is to make sure no other data is provided. Because if you set the bind type to string I can also set the value to "I'm not a number and I will break your query" and the code would accept it. Then MySql would throw the error (under normal circumstances, it could be hidden) and you would have no idea why your program is failing.