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11.13.1 Types of Numeric Values »
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11.13. Precision Math

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11.13.1. Types of Numeric Values
11.13.2. DECIMAL Data Type Changes
11.13.3. Expression Handling
11.13.4. Rounding Behavior
11.13.5. Precision Math Examples

MySQL 5.0 introduces precision math: numeric value handling that results in more accurate results and more control over invalid values than in earlier versions of MySQL. Precision math is based on two implementation changes:

  • The introduction of SQL modes in MySQL 5.0 that control how strict the server is about accepting or rejecting invalid data.

  • The introduction in MySQL 5.0.3 of a library for fixed-point arithmetic.

These changes have several implications for numeric operations:

  • More 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.

  • Improved 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 changes is that MySQL provides improved 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 5.0 handles numeric operations precisely. For information about using the sql_mode system variable to control the SQL mode, see Section 5.1.7, “Server SQL Modes”.