not null primary key,
name varchar(10) null,
indate datetime null,
tid int null,
key(tid),...not null primary key,
name varchar(10) null,
indate datetime null,
tid int null,
key(tid),...=b.tid and a.tid=2;
mysql> explain select * from tabname where tid=2;
+----+-------------...from tabname where tid=2 or tid is null;
+----+-------------+---------+-------------+---------...`tid` AS `tid` from `test`.