The easiest way to check to see if you use exactly one of your hole cards would be use the database field "holdem_hand_player_combinations.val_f_hole_cards_used", but that field isn't populated correctly in all cases.
An expression which does seem to work to count one pair hands, using exactly one of your holecards is:
sum( if[ holdem_hand_player_combinations.flg_f_1pair AND (holdem_hand_player_combinations.id_f_hand_strength=2 OR holdem_hand_player_combinations.id_f_hand_strength=3) AND holdem_hand_player_statistics.flg_f_saw, 1, 0 ] )
This counts only times you actually saw the flop, so to create the stat you'll need to divide by cnt_f_saw.
If you want to see how often you would have flopped a pair regardless of whether you folded preflop then you can remove the check for seeing the flop and divide by cnt_hands instead.
Another alternative would be to check:
(holdem_hand_player_detail.holecard_1 % 13 <> holdem_hand_player_detail.holecard_2 % 13) AND (holdem_hand_summary.card_1 % 13 <> holdem_hand_summary.card_2 % 13) AND (holdem_hand_summary.card_1 % 13 <> holdem_hand_summary.card_3 % 13) AND (holdem_hand_summary.card_2 % 13 <> holdem_hand_summary.card_3 % 13)
..which checks that the hole cards aren't the same rank, and that none of the flop cards are the same rank.
% means modulo (remainder).