Mysql in、exists、find_set_in

CREATE TABLE `www` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `year` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `season` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `amount` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

CREATE TABLE `shop` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `company_id` int(11) DEFAULT NULL,
  `shop_name` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `shop_pic` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

in通常是走索引的，当in后面的数据在数据表中超过30%及可能不走索引（全盘扫描）

#查询www表里的 年为1991、1992
select * from www where year in (1991,1992);

# select * from www where id = 1 条件作为bool值 true执行 false不执行
select * from www  where exists (select * from www where id = 1);

#FIND_IN_SET(查找的字段,所有字段);
#注意所有的字段是以逗号分割的 比如2,3,4 
#完整版 find_in_set(id,(2,3,4)) 
#select * from shop where FIND_IN_SET(id,(2,3,4));
select * from shop where FIND_IN_SET(id,(select season from www where id = 1));