Defines the Databases setup, i.e., JDBC driver information, JDBC URL, database type conversion, etc.


  • >= 3.1.3


  • General

    • jdbcDriver

      the comma-separated list of jdbc drivers to try loading

    • jdbcURL

      the JDBC URL to the database

  • Table creation


      database specific datatype, e.g., TEXT


      database specific datatype, e.g., INT


      database specific datatype, e.g., DOUBLE

  • Database flags

    • checkUpperCaseNames

      necessary if database turns column names into upper case ones, e.g., HSQLDB

    • checkLowerCaseNames (> 3.5.3)

      necessary if database turns column names into lower case ones, e.g., PostgreSQL

    • checkForTable (> 3.5.3)

      Checks whether the tables in the query are available in the meta-data of the JDBC Connection. Some tables, like pg_tables, exist but are not available through the meta-data

    • setAutoCommit

      setting for java.sql.Connection.setAutoCommit(boolean)

    • createIndex

      whether to create a primary key Key_IDX in the results table of an experiment

  • Special flags for DatabaseLoader/Saver (package weka.core.converters)

    • nominalToStringLimit (>= 3.4.1)

      beyond this limit, nominal columns are loaded as STRING attributes and no longer as NOMINAL ones

    • idColumn (>= 3.4.1)

      unique key in table that allows ordering for incremental loading

    • Keywords (> 3.5.8, > 3.6.0)

      lists all the reserved keywords of the current database type


    • KeywordsMaskChar (> 3.5.8, > 3.6.0)

      the character to append to attribute names/table names that would be interpreted as keywords by the database, in order to avoid exceptions when executing SQL commands defaut: _

Database type mapping

In order to import the data from database correctly into Weka, one has to specify what JDBC datatype corresponds to what Java SQL retrieval method. Here's an overview of how the Java types are mapped to Weka's attribute types:

Java type Java method Identifier Weka attribute type Version
String getString() 0 nominal
boolean getBoolean() 1 nominal
double getDouble() 2 numeric
byte getByte() 3 numeric
short getByte() 4 numeric
int getInteger() 5 numeric
long getLong() 6 numeric
float getFloat() 7 numeric
date getDate() 8 date
text getString() 9 string >3.5.5
time getTime() 10 string >3.5.8

In the props file one lists now the type names that the database returns and what Java type it represents (via the identifier), e.g.:


CHAR and VARCHAR are both String types, hence they are interpreted as String (identifier 0)

Note: in case database types have blanks, one needs to replace those blanks with an underscore, e.g., DOUBLE PRECISION must be listed like this:


See also