In Joiners, sometimes more slave records have the same values of corresponding fields of Join key. These slaves are called duplicates. If such duplicate slave records are allowed, all of them are parsed and joined with the master record if they match any. If the duplicates are not allowed, only one of them or at least some of them is/are parsed (if they match any master record) and the others are discarded.
Different Joiners allow to process slave duplicates in a different way. Here is a brief overview of how these duplicates are parsed and what can be set in these components or other tools:
Allow slave duplicates attribute is
included in the following Joiners (It can be
set to true or
false.):
ExtHashJoin
Default is false. Only the first record is processed, the others
are discarded.
ExtMergeJoin
Default is true. If switched to
false, only the last record is processed, the others
are discarded.
RelationalJoin
Default is false. Only the first record is processed, the others
are discarded.
SQL query attribute is included in DBJoin. SQL query allows to specify the exact number of slave duplicates explicitly.
LookupJoin parses slave duplicates according to the setting of used lookup table in the following way:
Simple lookup table has also the
Allow key duplicate attribute. Its
default value is true. If you uncheck the
checkbox, only the last
record is processed, the others are discarded.
DB lookup table allows to specify the exact number of slave duplicates explicitly.
Range lookup table does not allow slave duplicates. Only the first slave record is used, the others are discarded.
Persistent lookup table can work in two modes: with and without slave duplicates. See Range Lookup Table.
Aspell lookup table allows that all slave duplicates are used. No limitation of the number of duplicates is possible.