How to Auto-Increment a Column per Category Group in SQL?

SQL auto-increment column, grouped by category

Question:

How to create a SQL solution that automatically increments a column ("category_id") in each category while preserving the global sequence identifier ("id") and unique category constraints?

Answer:

It is generally not recommended to try what you describe as it may cause inconsistencies in a multi-user environment. Here are some reasons:

• Race condition: Multiple users accessing the database simultaneously may cause category_id values ​​to be duplicated or skipped.
• Transaction Issues: If an insert operation involves multiple queries, data consistency may not be ensured in the event of an error.

Recommended method:

• Remove category_id column: It provides no information that id and category themselves do not.
• Use id as the global identifier: It is itself serialized and auto-incremented.
• Generate category_id on the fly using row_number(): This function can assign sequential numbers within each category, giving the desired effect without the disadvantages of a separate category_id column.

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