mySQL UNION运算符的默认规则研究

/ 建立数据表 /
create table td_base_data( id int(10) not null auto_increment,userId int(10) default '0',primary key (`id`))ENGINE=MyISAM DEFAULT CHARSET=gbk;
create table td_base_data_20090527( id int(10) not null auto_increment,userId int(10) default '0',primary key (`id`))ENGINE=MyISAM DEFAULT CHARSET=gbk;
/ 插入模拟记录 /
insert into td_base_data(userId) values(1);
insert into td_base_data(userId) values(45);
insert into td_base_data(userId) values(45);
insert into td_base_data(userId) values(1);
insert into td_base_data(userId) values(45);
insert into td_base_data_20090527(userId) values(1);
insert into td_base_data_20090527(userId) values(45);
insert into td_base_data_20090527(userId) values(45);
insert into td_base_data_20090527(userId) values(1);
insert into td_base_data_20090527(userId) values(45);
insert into td_base_data_20090527(userId) values(45);
/ 查询测试 /
select count(userId) as umber from td_base_data where userId = '45';
/ 3 /
select count(userId) as umber from td_base_data_20090527 where userId = '45';
/ 4 /
select (select count(userId) from td_base_data where userId = '45') + (select count(userId) from td_base_data_20090527 where userId = '45') as umber;
/ 7 /
select count() from
(
select id from td_base_data where userId = '45'
union
select id from td_base_data_20090527 where userId = '45'
) as tx;
/ 4 /
select count() from
(
select from td_base_data where userId = '45'
union
select from td_base_data_20090527 where userId = '45'
) as tx;
/ 4 /
/ 证明在mysql中，union本身有剔除重复项的作用 /

/ 查询手册定义 /
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