Difference between numeric, float and decimal in SQL Server

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