SQL语句，使用like进行模糊查询时， 以下 会导致全表
扫描。
Aselect * from table_1 where username like ‘%yue%’
Bselect * from table_1 where username like ‘yue%’
Cselect * from table_1 where username like ‘wang%’
Dselect * from table_1 where username like ‘li%’