FIRST
For each parent record, returns the value from the first child record. The child records are looked at in natural unsorted order.
FIRST ( <field_name> [, Index [, <filter_query>] ] )
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field_name: The name of a field to return the value from.
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index: The index of the record to return.
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filter_query: A query string representing the domain of records to include.
Example: create a field on the CUSTOMER table that returns the product code of the second purchase that each customer bought in 1994.
FIRST( [PRODUCT.PRODUCT_CODE],2,[PRODUCT.PURCHASE YEAR] = "1994")