decimal and numeric (Transact-SQL)
Numeric data types that have fixed precision and scale.
For the decimal and numeric data types, SQL Server considers each specific combination of precision and scale as a different data type. For example, decimal(5,5) and decimal(5,0) are considered different data types.
In Transact-SQL statements, a constant with a decimal point is automatically converted into a numeric data value, using the minimum precision and scale necessary. For example, the constant 12.345 is converted into a numeric value with a precision of 5 and a scale of 3.
Converting from decimal or numeric to float or real can cause some loss of precision. Converting from int, smallint, tinyint, float, real, money, or smallmoney to either decimal or numeric can cause overflow.
By default, SQL Server uses rounding when converting a number to a decimal or numeric value with a lower precision and scale. However, if the SET ARITHABORT option is ON, SQL Server raises an error when overflow occurs. Loss of only precision and scale is not sufficient to raise an error.
When converting float or real values to decimal or numeric, the decimal value will never have more than 17 decimals. Any float value < 5E-18 will always convert as 0.