Видимо пользуетесь DBASE IV драйвером

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dBASE Data Types
The following table shows how dBASE data types are mapped to ODBC SQL data types. Note that not all ODBC SQL data types are supported.



[1] Valid only for dBASE version 5.x

Precision in dBASE III allows numbers with up to two-digit exponents and in dBASE IV numbers with up to three-digit exponents. Because numbers are stored as text, they are converted to numbers. If the number to convert does not fit in a field, unexplained results may occur.

While dBASE allows a precision and a scale to be specified with a NUMERIC data type, it is not supported by the ODBC dBASE driver. The ODBC dBASE driver always returns a precision of 15 and a scale of 0 for a NUMERIC data type.

A column created with the Numeric data type using the ODBC dBASE driver maps to the SQL_DOUBLE ODBC data type. Thus the data in this column is subject to rounding. This behavior is not the same as that of the NUMERIC data type in dBASE (type N), which is Binary Coded Decimal (BCD).
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У меня какой тип поля не выбирай при create table LONG ли DOUBLE - это всегда будет DECIMAL(20,5).