替換null:isnull(arg,value)
如:select isnull(price,0.0) from orders ,如果price為null的話，用0.0替換
與null比較: is not null，is null
如 select * from orders where price is null ,price等於null
如: select * from orders where price is not null ,price不等於null
Oracle
替換null: nvl(arg,value)
如: select nvl(price,0.0) form orders
與null比較: is not null，is null
如 select * from orders where price is null ,price等於null
如: select * from orders where price is not null ,price不等於null
