Difference between numeric, float and decimal in SQL Server

What are the differences between numeric, float and decimal datatypes and which should be used in which situations?

For any kind of financial transaction (e.g. for salary field), which one is preferred and why?

> use the float or real data types only if the precision provided by decimal (up to 38 digits) is insufficient

• Approximate numeric data types(see table 3.3) do not store the exact values specified for many numbers; they store an extremely close approximation of the value. (Technet)

• Avoid using float or real columns in WHERE clause search conditions, especially the = and <> operators. It is best to limit float and real columns to > or < comparisons. (Technet)

so generally choosing Decimal as your data type is the best bet if

• your number can fit in it. Decimal precision is 10E38[~ 38 digits]
• smaller storage space (and maybe calculation speed) of Float is not important for you
• exact numeric behavior is required, such as in financial applications, in operations involving rounding, or in equality checks. (Technet)

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1. Exact Numeric Data Types decimal and numeric - MSDN

• numeric = decimal (5 to 17 bytes)
  • will map to Decimal in .NET
  • both have (18, 0) as default (precision,scale) parameters in SQL server
  • scale = maximum number of decimal digits that can be stored to the right of the decimal point.
  • money(8 byte) and smallmoney(4 byte) are also Exact Data Type and will map to Decimal in .NET and have 4 decimal points (MSDN)

2. Approximate Numeric Data Types float and real - MSDN

• real (4 byte)
  • will map to Single in .NET
  • The ISO synonym for real is float(24)
• float (8 byte)
  • will map to Double in .NET

• All exact numeric types always produce the same result, regardless of which kind of processor architecture is being used or the magnitude of the numbers
• The parameter supplied to the float data type defines the number of bits that are used to store the mantissa of the floating point number.
• Approximate Numeric Data Type usually uses less storage and have better speed (up to 20x) and you should also consider when they got converted in .NET
• What is the difference between Decimal, Float and Double in C#
• Decimal vs Double Speed
• SQL Server - .NET Data Type Mappings (From MSDN)

main source : MCTS Self-Paced Training Kit (Exam 70-433): Microsoft® SQL Server® 2008 Database Development - Chapter 3 - Tables, Data Types, and Declarative Data Integrity Lesson 1 - Choosing Data Types (Guidelines) - Page 93

Guidelines from MSDN: Using decimal, float, and real Data

> The default maximum precision of numeric and decimal data types is 38. > In Transact-SQL, numeric is functionally equivalent to the decimal > data type. Use the decimal data type to store numbers with decimals > when the data values must be stored exactly as specified. > > The behavior of float and real follows the > IEEE 754 specification on approximate numeric data types. Because of the approximate nature of the float and real data types, do not use these data types when exact > numeric behavior is required, such as in financial applications, in > operations involving rounding, or in equality checks. Instead, use the > integer, decimal, money, or smallmoney data types. Avoid using float > or real columns in WHERE clause search conditions, especially the = > and <> operators. It is best to limit float and real columns to > or < > comparisons.

They Differ in Data Type Precedence

Decimal and Numeric are the same functionally but there is still data type precedence, which can be crucial in some cases.

SELECT SQL_VARIANT_PROPERTY(CAST(1 AS NUMERIC) + CAST(1 AS DECIMAL),'basetype')

The resulting data type is numeric because it takes data type precedence.

Exhaustive list of data types by precedence:

Reference link

Not a complete answer, but a useful link:

"I frequently do calculations against decimal values. In some cases casting decimal values to float ASAP, prior to any calculations, yields better accuracy. "

http://sqlblog.com/blogs/alexander_kuznetsov/archive/2008/12/20/for-better-precision-cast-decimals-before-calculations.aspx

Decimal has a fixed precision while float has variable precision.

EDIT (failed to read entire question): Float(53) (aka real) is a double-precision (64-bit) floating point number in SQL Server. Regular Float is a single-precision (32-bit) floating point number. Double is a good combination of precision and simplicty for a lot of calculations. You can create a very high precision number with decimal -- up to 136-bit -- but you also have to be careful that you define your precision and scale correctly so that it can contain all your intermediate calculations to the necessary number of digits.

The case for Decimal

What it the underlying need?

It arises from the fact that, ultimately, computers represent, internally, numbers in binary format. That leads, inevitably, to rounding errors.

Consider this:

0.1 (decimal, or "base 10") = .00011001100110011... (binary, or "base 2")

The above ellipsis [...] means 'infinite'. If you look at it carefully, there is an infinite repeating pattern (='0011')

So, at some point the computer has to round that value. This leads to accumulation errors deriving from the repeated use of numbers that are inexactly stored.

Say that you want to store financial amounts (which are numbers that may have a fractional part). First of all, you cannot use integers obviously (integers don't have a fractional part). From a purely mathematical point of view, the natural tendency would be to use a float. But, in a computer, floats have the part of a number that is located after a decimal point - the "mantissa" - limited. That leads to rounding errors.

To overcome this, computers offer specific datatypes that limit the binary rounding error in computers for decimal numbers. These are the data type that should absolutely be used to represent financial amounts. These data types typically go by the name of Decimal. That's the case in C#, for example. Or, DECIMAL in most databases.

Float is Approximate-number data type, which means that not all values in the data type range can be represented exactly.

Decimal/Numeric is Fixed-Precision data type, which means that all the values in the data type range can be represented exactly with precision and scale. You can use decimal for money saving.

Converting from Decimal or Numeric to float can cause some loss of precision. For the Decimal or Numeric data types, SQL Server considers each specific combination of precision and scale as a different data type. DECIMAL(2,2) and DECIMAL(2,4) are different data types. This means that 11.22 and 11.2222 are different types though this is not the case for float. For FLOAT(6) 11.22 and 11.2222 are same data types.

You can also use money data type for saving money. This is native data type with 4 digit precision for money. Most experts prefers this data type for saving money.

Reference 1 2 3

Although the question didn't include the MONEY data type some people coming across this thread might be tempted to use the MONEY data type for financial calculations.

Be wary of the MONEY data type, it's of limited precision.

There is a lot of good information about it in the answers to this Stackoverflow question:

https://stackoverflow.com/questions/582797/should-you-choose-the-money-or-decimalx-y-datatypes-in-sql-server