sql server

　　替换null:isnull(arg,value)

　　如:select isnull(price,0.0) from orders ,如果price为null的话，用0.0替换

　　与null比较: is not null，is null

　　如 select * from orders where price is null ,price等于null

　　如: select * from orders where price is not null ,price不等于null

　　Oracle

　　替换null: nvl(arg,value)

　　如: select nvl(price,0.0) form orders

　　与null比较: is not null，is null

　　如 select * from orders where price is null ,price等于null

　　如: select * from orders where price is not null ,price不等于null