##
Chapter 24. Precision Math

MySQL 5.1 provides support for precision math: numeric
value handling that results in extremely accurate results and a high
degree control over invalid values. Precision math is based on these
two features:

These features have several implications for numeric operations:

**Precise calculations**: For
exact-value numbers, calculations do not introduce
floating-point errors. Instead, exact precision is used. For
example, a number such as `.0001`

is treated as
an exact value rather than as an approximation, and summing it
10,000 times produces a result of exactly `1`

,
not a value that merely “close” to 1.

**Well-defined rounding behavior**:
For exact-value numbers, the result of
`ROUND()`

depends on its argument, not on
environmental factors such as how the underlying C library
works.

**Platform independence**:
Operations on exact numeric values are the same across different
platforms such as Windows and Unix.

**Control over handling of invalid
values**: Overflow and division by zero are detectable
and can be treated as errors. For example, you can treat a value
that is too large for a column as an error rather than having
the value truncated to lie within the range of the column's data
type. Similarly, you can treat division by zero as an error
rather than as an operation that produces a result of
`NULL`

. The choice of which approach to take is
determined by the setting of the `sql_mode`

system variable.

An important result of these features is that MySQL 5.1
provides a high degree of compliance with standard SQL.

The following discussion covers several aspects of how precision
math works (including possible incompatibilities with older
applications). At the end, some examples are given that demonstrate
how MySQL 5.1 handles numeric operations precisely. For
information about using the `sql_mode`

system
variable to control the SQL mode, see
Section 5.2.5, “The Server SQL Mode”.