MySQL: Very slow query, not using index, no joins involved
You may want the following indexes

ALTER TABLE tran.member ADD INDEX cardnbr_mbr_nbr_ndx (cardnbr,mbr_nbr);
ALTER TABLE tran.member ADD INDEX mbr_nbr_cardnbr_ndx (mbr_nbr,cardnbr);


Then, you could queries like these

Each Member and All Cards for That Member

SELECT mbr_nbr MemberNumber,GROUP_CONCAT(cardnbr) Cards
FROM tran.member GROUP BY mbr_nbr;


Each Card and All Members Associated with that Card

SELECT cardnbr CardNumber,GROUP_CONCAT(mbr_nbr) Members
FROM tran.member GROUP BY cardnbr;


So, that the size of tran.member does not slow things down, try creating a temp table for the purpose of gathering these two columns

DROP TABLE IF EXISTS tran.adhoc_members_cards;
CREATE TABLE tran.adhoc_members_cards
SELECT mbr_nbr,cardnbr FROM tran.member WHERE 1=2;
ALTER TABLE tran.adhoc_members_cards ENGINE=MyISAM;
ALTER TABLE tran.adhoc_members_cards ADD INDEX cardnbr_mbr_nbr_ndx (cardnbr,mbr_nbr);
ALTER TABLE tran.adhoc_members_cards ADD INDEX mbr_nbr_cardnbr_ndx (mbr_nbr,cardnbr);
ALTER TABLE tran.adhoc_members_cards DISABLE KEYS;
INSERT INTO tran.adhoc_members_cards
SELECT mbr_nbr,cardnbr FROM tran.memberl
SELECT mbr_nbr,cardnbr FROM tran.member WHERE 1=2;
ALTER TABLE tran.adhoc_members_cards ENABLE KEYS;


Now, you can run the two queries I mentioned like this:

SELECT mbr_nbr MemberNumber,GROUP_CONCAT(cardnbr) Cards
FROM tran.adhoc_members_cards GROUP BY mbr_nbr;
SELECT cardnbr CardNumber,GROUP_CONCAT(mbr_nbr) Members
FROM tran.adhoc_members_cards GROUP BY cardnbr;


Search for the cardmember you want as well:

SELECT cardnbr CardNumber,GROUP_CONCAT(mbr_nbr) Members
FROM tran.adhoc_members_cards WHERE cardnbr = 000099999930;