Question

I need to generate encoding String for each item I inserted into the database. for example:

x00001 for the first item
x00002 for the sencond item
x00003 for the third item

The way I chose to do this is counting the rows. Before I insert the third item, I count against the database, I know there’re already 2 rows, so the next encoding is ended with 3.
But there is a problem. If I delete the second item, the forth item will not be the x00004,but x00003.

I can add additional columns to table, to store the next encoding, I don’t know if there’s other better solutions ?

Answer 1

What you want is SELECT MAX(id) or SELECT MAX(some_function(id)) inside the transaction.

As suggested in Oded’s answer a lot of databases have their own methods of providing sequences which are more efficient and depending on the DBMS might support non numeric ids.

Also you could have id broken down into Y and 00001 as separate columns and having both columns make up primary key; then most databases would be able to provide the sequence.

However this leads to the question if your primary key should have a meaning or not; Y suggest that there is some meaning in the part of the key (otherwise you would be content with a plain integer id).