Joe Celko's SQL for Smarties, 3rd Edition

3.2. Numeric Type Conversion

There are a few surprises in converting from one numeric type to another. The SQL Standard left it up to the implementation to answer a lot of basic questions, so the programmer has to know his package.

3.2.1. Rounding and Truncating

When an exact or approximate numeric value is assigned to an exact numeric column, it may not fit. SQL says that the database engine will use an approximation that preserves leading significant digits of the original number after rounding or truncating. The choice of whether to truncate or round is implementation-defined, however. This can lead to some surprises when you have to shift data among SQL implementations, or shift storage values from a host language program into an SQL table. ...