数据库中的主键如何工作？

Question

I'm using a datagridview to display the records of a database. I also have a form that collects new data to be added to the database too. What I want to know is if the primary key is generated automatically or not once the data is inserted in the database using dataAdapter-DataSets.

I have a method in a class that handles all the tables for the different datagridviews. The client method looks like this.

public void AddNewClient (Client client)
        {
            try
            {
                DataRow dr = dataSet.Tables["Clients"].NewRow ();
                dr[1] = client.firstName;
                dr[2] = client.lastName;
                dr[3] = client.MI;
                dr[4] = client.address;
                dr[5] = client.city;
                dr[6] = client.state;
                dr[7] = client.zipcode;
                dr[8] = client.homePhone;
                dr[9] = client.cellPhone;
                dr[10] = client.otherPhone;
                dr[11] = client.faxNumber;
                dr[12] = client.email;
                dataSet.Tables["Clients"].Rows.Add (dr);

                adapterClients.Update (dataSet, "Clients");
            }
            catch (Exception e) { throw e; }
        } 

The dataSet.Tables["Clients"] is bind to the DataGridViewClients. When the new record displays in the DataGridViewClients there's no ClientID value. However, when I restart the application and the DataGridViewClients is displayed I can see how a new value to ClientID has been assigned. So I wonder is this something that is done automatically by SQL server or ....? I mean I don't really know. :)

Answer 1

The person who created the database decided at that time how the PRIMARY KEY works.

One possible (and common) way to declare a PRIMARY KEY is to make it an INTEGER column for which the database engine itself will create values when new rows are INSERTed. In SQL Server this kind of column is called an IDENTITY (other products might call it an AUTOINC, AUTONUMBER, or GENERATOR column). This is probably what's happening in your database.

Another technique is to use an INTEGER field in which the application programmer is responsible for creating a unique value. This is less practical because you have to deal with collisions between multiple simultaneous users.

Finally, some designers try to identify one or more columns of meaningful data that are guaranteed to be unique for each record in the system and declare that combination of columns to be the PRIMARY KEY. In this case there's no separate numbering system for the records; the PRIMARY KEY is made up of "real" data.

Answer 2

The ClientID is probably an identity column . That means the database automatically assigns a new number to each row. Even if you wanted to, you would not be allowed to set the value of the identity column yourself.

An identity column is often a primary key, but this is not always true. A primary key can be made up of any set of columns that uniquely identifies a row. It's easy to see that an "auto-number" or identity column meets this requirement.

You can list all identity columns in your database with the following query:

select  t.name
,       c.name
from    sys.tables t
join    sys.columns c
on      c.object_id = t.object_id
where   c.is_identity = 1