CONNECT DBF Table Type

Overview

A table of type DBF is physically a dBASE III or IV formatted file (used by many products like dBASE, Xbase, FoxPro etc.). This format is similar to the FIX type format with in addition a prefix giving the characteristics of the file, describing in particular all the fields (columns) of the table.

Because DBF files have a header that contains Meta data about the file, in particular the column description, it is possible to create a table based on an existing DBF file without giving the column description, for instance:

create table cust engine=CONNECT table_type=DBF file_name='cust.dbf';

To see what CONNECT has done, you can use the DESCRIBE or SHOW CREATE TABLE commands, and eventually modify some options with the ALTER TABLE command.

The case of deleted lines is handled in a specific way for DBF tables. Deleted lines are not removed from the file but are "soft deleted" meaning they are marked as deleted. In particular, the number of lines contained in the file header does not take care of soft deleted lines. This is why if you execute these two commands applied to a DBF table named tabdbf:

select count(*) from tabdbf;
select count(*) from tabdbf where 1;

They can give a different result, the (fast) first one giving the number of physical lines in the file and the second one giving the number of line that are not (soft) deleted.

The commands UPDATE, INSERT, and DELETE can be used with DBF tables. The DELETE command marks the deleted lines as suppressed but keeps them in the file. The INSERT command, if it is used to populate a newly created table, constructs the file header before inserting new lines.

Note: For DBF tables, column name length is limited to 11 characters and field length to 256 bytes.

Conversion of dBASE Data Types

CONNECT handles only types that are stored as characters.

For the N numeric type, CONNECT converts it to TYPE_DOUBLE if the decimals value is not 0, to TYPE_BIGINT if the length value is greater than 10, else to TYPE_INT.

For M, B, and G types, CONNECT just returns the DBT number.

Reading soft deleted lines of a DBF table

It is possible to read these lines by changing the read mode of the table. This is specified by an option READMODE that can take the values:

For example, to read all lines of the tabdbf table, you can do:

alter table tabdbf option_list='Readmode=1';

To come back to normal mode, specify READMODE=0.