FilterOnce Operator

The FilterOnce operator can be leveraged to filter out unique expressions from an input source.

What	Type
Syntax	FilterOnce(expression)
expression	string
Return type	bool

First, the expression is evaluated to a string; the operator keeps a map of "seen" values and compares the expression string with what it already knows.

• Case a) The operator has not yet seen the value: In this case, FilterOnce returns true, and stores the value for future reference
• Case b) The operator has already seen the value. The operator returns false.

The FilterOnce operator should only be used inside a <SourceFilter> tag (see Config File Documentation).

Attention: Please note that this only filters for unique expressions; this does not mean that all other fields which you may or may not output in your field mapping also match among all records where the expression match. The values of the other fields you output will be taken from the record current in Case b) above. This means, it is not a 100% equivalent of SQLs SELECT DISTINCT feature.

Examples:

Filter	Description
<SourceFilter>FilterOnce($LastName)</SourceFilter>	Filters for unique $LastName values
<SourceFilter>FilterOnce($FirstName + " " + $LastName</SourceFilter>	Filters for unique full names

See also FilterDuplicate Operator.