嵌套查询

select name,age from person where  age > (select age from person where name = '曹操')   //从person中查询name和age，前提条件是年龄要大于姓名=曹操的年龄;
select name from person where countryid in (select countryid from country  where countryname = '魏国')  //从person表中查询name值，前提条件countryid是countryname = '魏国'的countryid
select name from person where countryid = some (select countryid from country  where countryname = '魏国') //从person表中查询name值，前提条件countryid在XX中
select name from person where countryid > all　(select countryid from country  where countryname = '魏国') //countryid大于countryname='魏国'的数据
SELECT * FROM Person WHERE  exists (SELECT * FROM Person WHERE Person_Id = 100 )