How to search a chess database for exchange sacrifices? I am looking for a way within any of the common chess packages to search for a specific maneuver - rook takes bishop that is protected  (i.e. an exchange sacrifice)? I've heard of CQL, but never used, so I would be curious if it supports something like this.
Chess Query Language does do this very well. The following short .cql file works like a charm to find all games in input.pgn in which one side is up an exchange (while allowing a difference in the number of pawns for each side) for 4 straight moves (picked so as to rule out, say, situations in which an equal sequence of trades happens to begin with a rook taking a minor piece, or other such temporary pseudo-sacrifices).

(match
    :pgn input.pgn
    :output output.pgn
    (position
    :sequence(
        (position :powerdifference [Rr] -5 -5 :powerdifference [Qq] 0 0 :powerdifference [BbNn] 3 3)
        (position :powerdifference [Rr] -5 -5 :powerdifference [Qq] 0 0 :powerdifference [BbNn] 3 3)
        (position :powerdifference [Rr] -5 -5 :powerdifference [Qq] 0 0 :powerdifference [BbNn] 3 3)
        (position :powerdifference [Rr] -5 -5 :powerdifference [Qq] 0 0 :powerdifference [BbNn] 3 3)
    )
    :flipcolor
    )
)


Standard point values for pieces are used, so the :powerdifference [Rr] -5 -5 condition means that Black has one more rook than White, while :powerdifference [BbNn] 3 3 guarantees that White has exactly one more minor piece than Black. The :flipcolor at the end does the same search with the roles reversed. By default, CQL inserts a "MATCH" comment in the games of the resulting output.pgn at the positions that warrant their inclusion, so you can quickly locate the exchange sacs.