update index after a delete operation

Question

I have a Customer Table that has a CustIndex column. From the UI a User can delete a customer that was for example at Index 3. What I need to do then determine if there was a Customer with an Index at 4 and renumber it to be 3, (so basically keep the sequence).

At the minute I just have some code like below:

    private void UpdateCustomerIndexes(IList<CustomerDTO> customers)
    {
        var customerIndexes = customers.Select(c => new { c.customerId, c.customerIndex });

        //unitIndexes.Select(u => u.)
    }

I am keeping wanting to keep the CustomerId so if I need to update the DB I can say update customer set index = 3 where customerid = 4 for example.

So if I had 5 customer on the UI and I delete Customer at Index 5

If I set a breakpoint on customerIndexes, the data is:

{ CustomerId = 33, CustomerIndex = 1 },
{ CustomerId = 34, CustomerIndex = 2 },
{ CustomerId = 35, CustomerIndex = 3 },
{ CustomerId = 36, CustomerIndex = 4 }

So, in this case, the CustomerIndex is in sequence so there is nothing to do. However, lets say now I deleted CustomerIndex 2 on the UI. The data would now look like:

{ CustomerId = 33, CustomerIndex = 1 },
{ CustomerId = 35, CustomerIndex = 3 },
{ CustomerId = 36, CustomerIndex = 4 }

Now the Customer Index is out of sequence and I need to renumber CustomerIndex 3 to 2 and CustomerIndex 4 to 3 and then save this to the DB so I can say make the update statements setting the Index to the new value for that CustomerId.

I know my CustomerIndex will never go above 50 so would it be an idea to use something like:

var list = new List<int>(new[] { customerIndexes.Select(c => c.customerIndex) });
var result = Enumerable.Range(0, 50).Except(list);

However, I think this will only tell me if a number in sequence is missing - I am missing how to save which index needs to be updated for which customerid or do nothing in the case when they are in sequence

Answer 1

After you delete the record with CustomerIndex of 2, run the following SQL:

UPDATE TableName SET CustomerIndex = CustomerIndex - 1 WHERE CustomerIndex > 2

This will ensure the CustomerIndex of all 'greater' records are reduced by 1 to compensate.

Similarly, if a customer deletes CustomerIndex of 5, change both references to 2 above to 5.