create table t1 (t1 char(3) primary key);
insert into t1 values("ABC");
insert into t1 values("ABA");
insert into t1 values("AB%");
select * from t1 where t1="ABC";
t1
ABC
select * from t1 where t1="ABCD";
t1
select * from t1 where t1 like "a_\%";
t1
AB%
describe select * from t1 where t1="ABC";
table	type	possible_keys	key	key_len	ref	rows	Extra
t1	const	PRIMARY	PRIMARY	3	const	1	
describe select * from t1 where t1="ABCD";
Comment
Impossible WHERE noticed after reading const tables
drop table t1;