sql子查询嵌套规则 SQL里面的嵌套查询语句怎么写？

SQL里面的嵌套查询语句怎么写？1，简单子查询；select name，age from person where age > ( select age from person wh

SQL里面的嵌套查询语句怎么写？

1，简单子查询；select name，age from person where age > ( select age from person where name = "孙权")2，in嵌套查询；select name from person where countryid in ( select countryid from country where countryname = "魏国")3，some嵌套查询select name from person where countryid = some 　　　　　　--用等号和以下查询到的值比较，如果与其中一个相等，就返回( select countryid from country where countryname = "魏国")4，all嵌套查询select name from person where countryid > all　　 --当countryid大于以下返回的所有id，此结果才为True，此结果才返回( select countryid from country where countryname = "魏国")5，exits嵌套查询SELECT * FROM PersonWHERE exists( SELECT 1 --SELECT 0 SELECT NULL 返回结果都一样，因为这三个子查询都有结果集返回，因此总是True SELECT * FROM Person照常执行)　但是如果子查询中因为加了条件而没有结果集返回，则主语句就不执行了：SELECT * FROM PersonWHERE exists( SELECT * FROM Person WHERE Person_Id = 100 --如果不存在Person_Id的记录，则子查询没有结果集返回，主语句不执行)