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