如何确保用户不会尝试输入已经存在的主键ID？

Question

I have an application that adds pets to an SQL database.

Currently the user must know what PetIDs are existing and so must know what is available to be added. If the user tries to enter an already existing ID the program gives an error.

Im thinking i need the PetID (top text box) value to be automatically decided upon page load, with a value which wont clash with an already existing value...

Can someone help? i have no idea how to do this

I need the page_load to automatically search the SQL database for PetIDs that are available, pick the one with the lowest value and have it in the text box automatically, ready for the user (so the user wont have to worry about picking one which isn't already taken)...

How can i have an ID which is available, waiting in the top text box, upon page load? also make it so the user cannot attempt to change it.

Answer 1

You can create the PetId as an Identity Column as below:

[PetId] [int] IDENTITY(10,1) NOT NULL

Thus you will not have to add it manually, each time you add a record of Pet, the PetId will be generated automatically and that will be unique. So, no clashing of PetId will occur.

Here 10 represents the first Id number and 1 represents how your PetIds will be incremented.

Regards, Pratik

Answer 2

It is usual to enter your data and return the ID after the record is added to the database (using the Identity Insert on the Id column).

If you want the next number to be displayed BEFORE the data is created then use an integer column for ID without the Identity Insert option and create a SQL Server sequence. Create a stored procedure to return the next sequence number to show in your creation page

Answer 3

One option would be to use a sequence:

CREATE SEQUENCE pet_id_seq START WITH 7

I'm starting it with 7 because it is the first available pet_id

Then, on page load, get the next value for the sequence by executing the following query:

SELECT (NEXT VALUE FOR pet_id_seq) AS next_pet_id

And show the value returned by the query on the pet_id text box.

This approach has one disadvantage, it produce gaps, if you open the page, but you don't actually insert the pet row, the id is lost forever. But I think you should don't care, there are many numbers, you are not going to exhaust them.

Another option is to use the following query to fill the pet_id text box:

SELECT MAX(pet_id) + 1 AS next_pet_id FROM pets

But this one has another disadvantage. It doesn't work well on concurrent scenarios. If two users open the create pet page at the same time they get the same next_pet_id and one of them will get a primary key violation error.

Answer 4

Set the id column to auto increment and don't let the user insert it, let the database decide it for itself. Or you can change the id column type and use Guid.NewGuid() to generate a new id.