英语翻译
9.
Although primary keys are notactually required,most database designers en sure that every table they create has a primary key so that future datamanipulation is possible and manageable.Any column in a table can be established as the primary key,as long asit meets the following conditions:Notwo rows can have the same primary key value.Every row must have a primary key value (primary key columns may notallow NULL values).Values in primarykey columns can never be modified or updated.our ATM card at a cash machine,you are using databases for PIN number verification and balance checking.