SQL Server 的自动增量功能

Question

I have created a table named as ABC. It has three columns which are as follows:-

The column number_pk (int) is the primary key of my table in which I have made the auto increment feature on for that column.

Now I have deleted two rows from that table say Number_pk= 5 and Number_pk =6 .

The table which I get now is like this:-

Now if I again enter two new rows in this table with the same value I get the two new Number_pk starting from 7 and 8 ie,

My question is that what is the logic behind this since I have deleted the two rows from the table. I know that a simple answer is because I have set the auto increment on for the primary key of my table. But I want to know is there any way that I can insert the two new entries starting from the last Number_pk without changing the design of my table?

And how the SQL Server manage this record since I have deleted the rows from the database??

Answer 1

Although you should not do this until their is a specific requirement.

1.) Get the max id:

Declare @id int
Select @id = Max(Number_pk) From ABC
SET @id = @id + 1;

2.) And reset the Identity Column:

DBCC CHECKIDENT('ABC', RESEED, @id)

DBCC CHECKIDENT (Transact-SQL)

Answer 2

The logic is guaranteeing that the generated numbers are unique. An ID field does not neccessarily have to have a meaning, but rather is most often used to identify a unique record, thus making it easier to perform operations on it.

If your database is designed properly, the deleted ID numbers would not have been possible to delete if they were referenced by any other tables in a foreign key relationship, thus preventing records from being orphaned in that way.

If you absolutely want to have entries sequences, you could consider issuing a RESEED , but as suggested, it would not really give you much advantages.

The identity record is "managed" because SQL Server will keep track of which numbers have been issued, regardless of whether they are still present or not.

Should you ever want to delete all records from a table, there are two ways to do so (provided no foreign key relatsons exist):

DELETE FROM Table

DELETE just removes the records, but the next INSERTED value will continue where the ID numbering left of.

TRUNCATE TABLE

TRUNCATE will actually RESEED the table, thus guaranteeing it starts again at the value you originally specified (most likely 1).