Since I came from a VB background, I get confused when I encounter these data types in SQL Server.  In VB if I need to use a variable that can hold a number with digits to the right of the decimal point, I declare the variable as single or double.  In SQL Server, you have decimal, numeric, float and real to choose from.  So what is the difference between them?

Decimal and numeric

Decimal and numeric are called exact numerics in SQL Server because they have fixed precision and scale. 

Precision is the maximum total number of digits that can be stored, both to the left and to the right of the decimal point.  The value for precision can be from 1 to 38.

Scale is the maximum number of digits that can be stored to the right of the decimal point.  The value for scale can be from 0 to precision.

If you don’t specify the precision and scale when you declare the variable as decimal or numeric, by default it will have a precision of 18 digits and a scale of 0 digit.  It’s like declaring it as:

decimal (18, 0)

Since decimal and numeric are both the same, either numeric or decimal will do.

If you are concerned with storage, then use a precision of 9 digits or less and this will take only 5 bytes, the minimum for a decimal or numeric.  A precision of 10-19 digits will give you 9 bytes; 20-28 digits will give you 13 bytes; and 29-38 digits will give you 17 bytes.

Float and real

Float and real are called approximate numerics and are used for floating point numeric data (whatever that means).  Since decimal or numeric will serve my purpose if all I want is a number that can store digits to the right of the decimal point, then I don’t need to bother with float and real unless I am working on a mathematical application.