More COLUMN_LIST Component Fields in Talend Open Studio

In the XML descriptor for a Talend Open Studio custom component, COLUMN_LIST is used to present a list of the component schema's columns in a drop down on the Component View.  PREV_COLUMN_LIST and LOOKUP_COLUMN_LIST are variations.

 
COLUMN_LIST

A PARAMETER element with a FIELD attribute equal to "COLUMN_LIST" will display the columns from the schema associated with the component.  This is this most common COLUMN_LIST field type used in the standard components.

PREV_COLUMN_LIST

A PARAMETER element with a FIELD attribute equal to "PREV_COLUMN_LIST" will display the columns from the main input schema.  The main input schema is defined as the metadata associated with the single FLOW CONNECTOR.

A tFilterRow Using a PREV_COLUMN_LIST

 For example, a tFilterRow will define a set of filtering rules using columns from the input flow.

LOOKUP_COLUMN_LIST

A PARAMETER element with a FIELD attribute equal to "LOOKUP_COLUMN_LIST" will display the columns from the alternate input schemas.  The alternate input schemas are defined as the metadata associated with all the flows identified as LOOKUP CONNECTORs.  This list of columns combines spans several schemas.

A tJoin Using a LOOKUP_COLUMN_LIST (and PREV_COLUMN_LIST)

tJoin uses both a PREV_COLUMN_LIST -- to capture the main flow -- and a LOOKUP_COLUMN_LIST to create a join between two data sources.

COLUMN_LIST is a widely used FIELD in the standard Talend component library.  However, there are occasions where you'll want to have the user select a column from a different schema.  PREV_COLUMN_LIST and LOOKUP_COLUMN_LIST are two FIELD types that will do this for incoming connections.