如何使表中的某个域的值唯一？
数据表中某个域，里面的值只能是“true”或者是“false”，同时只能让一条记录是“true”，其他都是“false”

在MySql中是如何做到的，谢谢!


[解决办法]


见如下例子。

SQL code

mysql> create table ta (    ->  discountid int auto_increment primary key ,    ->  `selected` BOOL    -> );Query OK, 0 rows affected (0.09 sec)mysql>mysql> delimiter |mysql>mysql> CREATE TRIGGER ta_bi BEFORE INSERT ON ta    ->   FOR EACH ROW BEGIN    ->          DECLARE bExist int default 0;    ->          select count(*) into bExist from ta where selected;    ->          if bExist>0 and new.selected then    ->                  set new.discountid=1;    ->          end if;    ->   END;    -> |Query OK, 0 rows affected (0.08 sec)mysql>mysql> delimiter ;mysql> insert into ta values (null,true);Query OK, 1 row affected (0.06 sec)mysql> insert into ta values (null,true);ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'mysql> insert into ta values (null,false);Query OK, 1 row affected (0.06 sec)mysql> insert into ta values (null,false);Query OK, 1 row affected (0.08 sec)mysql> insert into ta values (null,true);ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'mysql> select * from ta;+------------+----------+| discountid | selected |+------------+----------+|          1 |        1 ||          2 |        0 ||          3 |        0 |+------------+----------+3 rows in set (0.00 sec)mysql>