use test;
drop table if exists tc1;
create table tc1(c1 int not null primary key, c2 varchar(11));
insert into tc1 values (1, 'aaa');
insert into tc1 values (2, 'bbbb');
insert into tc1 values (3, 'ccccc');
create index idxc2 on tc1(c2);
alter table tc1 add index idxc3(c1,c2);
select * from tc1;
c1	c2
1	aaa
2	bbbb
3	ccccc
show index from tc1;
Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment	Visible	Expression
tc1	0	PRIMARY	1	c1	A	NULL	NULL	NULL		BTREE	available		YES	NULL
tc1	1	idxc2	1	c2	A	NULL	NULL	NULL	YES	BTREE	available		YES	NULL
tc1	1	idxc3	1	c1	A	NULL	NULL	NULL		BTREE	available		YES	NULL
tc1	1	idxc3	2	c2	A	NULL	NULL	NULL	YES	BTREE	available		YES	NULL
truncate table tc1;
select * from tc1;
c1	c2
show index from tc1;
Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment	Visible	Expression
tc1	0	PRIMARY	1	c1	A	NULL	NULL	NULL		BTREE	available		YES	NULL
tc1	1	idxc2	1	c2	A	NULL	NULL	NULL	YES	BTREE	available		YES	NULL
tc1	1	idxc3	1	c1	A	NULL	NULL	NULL		BTREE	available		YES	NULL
tc1	1	idxc3	2	c2	A	NULL	NULL	NULL	YES	BTREE	available		YES	NULL
drop table tc1;