This is the official manual for the latest Org-mode release.

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3.5.7 Lookup functions

Org has three predefined Emacs Lisp functions for lookups in tables.

(org-lookup-first VAL S-LIST R-LIST &optional PREDICATE)
Searches for the first element S in list S-LIST for which
          (PREDICATE VAL S)

is t; returns the value from the corresponding position in list R-LIST. The default PREDICATE is equal. Note that the parameters VAL and S are passed to PREDICATE in the same order as the corresponding parameters are in the call to org-lookup-first, where VAL precedes S-LIST. If R-LIST is nil, the matching element S of S-LIST is returned.

(org-lookup-last VAL S-LIST R-LIST &optional PREDICATE)
Similar to org-lookup-first above, but searches for the last element for which PREDICATE is t.
(org-lookup-all VAL S-LIST R-LIST &optional PREDICATE)
Similar to org-lookup-first, but searches for all elements for which PREDICATE is t, and returns all corresponding values. This function can not be used by itself in a formula, because it returns a list of values. However, powerful lookups can be built when this function is combined with other Emacs Lisp functions.

If the ranges used in these functions contain empty fields, the E mode for the formula should usually be specified: otherwise empty fields will not be included in S-LIST and/or R-LIST which can, for example, result in an incorrect mapping from an element of S-LIST to the corresponding element of R-LIST.

These three functions can be used to implement associative arrays, count matching cells, rank results, group data etc. For practical examples see this tutorial on Worg.