CREATE TABLE test (id int(11) NOT NULL AUTO_INCREMENT,aid varchar(20) NOT NULL DEFAULT '' COMMENT 'aid',bid varchar(20) NOT NULL DEFAULT '' COMMENT 'bid',cid varchar(20) NOT NULL DEFAULT '' COMMENT 'cid',
PRIMARY KEY (id),
KEY abc (aid,bid,cid)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

INSERT INTO test VALUES('a01','b01','c01');
INSERT INTO test VALUES('a02','b02','c02');
INSERT INTO test VALUES('a03','b03','c03');

UPDATE test SET aid = 'a01', bid = 'b01', cid = 'c01' WHERE id = 1;
UPDATE test SET aid = 'a02', bid = 'b02', cid = 'c02' WHERE id = 2;
UPDATE test SET aid = 'a03', bid = 'b02', cid = 'c03' WHERE id = 3;

组合结果为 abc ab ac bc 四种组合结果

abc 能用到索引

explain select * from test where aid='a01' and bid='b01' and cid='c01';

bca能用到索引

explain select * from test where bid='b01' and cid='c01'and aid='a01';

cab能用到索引

explain select * from test where cid='c01'and aid='a01' and bid='b01';

ab 能用到索引

explain select * from test where aid='a01' and bid='b01';

ac 能用到索引

explain select * from test where aid='a01' and cid='c01';

ba 能用到索引

explain select * from test where bid='b01' and aid='a01';

ca 能用到索引

explain select * from test where cid='c01' and aid='a01';

bc 不能用到索引

explain select * from test where bid='b01' and cid='c01';

b 用不到索引

explain select * from test where bid='b01';

c 用不到索引

explain select * from test where cid='c01';

cb 用不到索引

explain select * from test where cid='c01' and bid='b01';

结论：
复合索引(abc) 所有情况的罗列，bca和 cab 也能用到索引是mysql优化器优化的结果，最根本的还是最左原则
abc 能用到索引
bca 能用到索引
cab 能用到索引
ab 能用到索引
ac 能用到索引
ba 能用到索引
ca 能用到索引
bc 用不到索引
b 用不到索引
c 用不到索引
cb 用不到索引