The DECIMAL and NUMERIC types store exact numeric data values. These types are used when it is important to preserve exact precision, for example with monetary data. In MySQL, NUMERIC is implemented as DECIMAL, so the following remarks about DECIMAL apply equally to NUMERIC.

Through version 4.1, MySQL stores DECIMAL values as strings, rather than in binary format. One character is used for each digit of the value, the decimal point (if the scale is greater than 0), and the “-” sign (for negative numbers).

In a DECIMAL column declaration, the precision and scale can be (and usually is) specified; for example:

salary DECIMAL(5,2)

In this example, 5 is the precision and 2 is the scale. The precision represents the number of significant digits that are stored for values, and the scale represents the number of digits that can be stored following the decimal point.

Standard SQL requires that DECIMAL(5,2) be able to store any value with five digits and two decimals, so values that can be stored in the salary column range from -999.99 to 999.99. In versions up to and including 4.1, MySQL varies from this limit in two ways due to the use of string format for value storage:

In standard SQL, the syntax DECIMAL(M) is equivalent to DECIMAL(M,0). Similarly, the syntax DECIMAL is equivalent to DECIMAL(M,0), where the implementation is permitted to decide the value of M. As of MySQL 3.23.6, both of these variant forms of DECIMAL syntax are supported. The default value of M is 10. Before 3.23.6, M and D both must be specified explicitly.

If the scale is 0, DECIMAL values contain no decimal point or fractional part.

The maximum range of DECIMAL values is the same as for DOUBLE, but the actual range for a given DECIMAL column can be constrained by the precision or scale for a given column. When such a column is assigned a value with more digits following the decimal point than are permitted by the specified scale, the value is converted to that scale. (The precise behavior is operating system-specific, but generally the effect is truncation to the permissible number of digits.)