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.

As of MySQL 5.0.3, DECIMAL values are stored in binary format. Previously, they were stored as strings, with one character used for each digit of the value, the decimal point (if the scale is greater than 0), and the “-” sign (for negative numbers). See Section 12.17, “Precision Math”.

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. MySQL enforces this limit as of MySQL 5.0.3. Before 5.0.3, on the positive end of the range, the column could actually store numbers up to 9999.99. (For positive numbers, MySQL 5.0.2 and earlier used the byte reserved for the sign to extend the upper end of the range.)

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. MySQL supports both of these variant forms of DECIMAL syntax. The default value of M is 10.

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

The maximum number of digits for DECIMAL is 65 (64 from MySQL 5.0.3 to 5.0.5). Before MySQL 5.0.3, 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.)