Golang & Postgres Interaction - Subtract 1 From int64 Table Every Query

Question

I'm learning to create HTTP API's,

I'm creating a dummy API credit system, as that's what I'm interested in specifically.

This is what i came up with: (I'm using PQ Driver)

if !dummy.creds <= 0 {
    c.JSON(404, gin.H{
        "success": false,
        "message": "No Credits!",
    })
    return
}

However, let's say i edit the SQL table to say 50 Credits, How do subtract 1 credit everytime the dummy API is queried?

So for example, if i Query the API and it returns the successful dummy data (in JSON) from the SQL table, how can i subtract one credit, so that i only have 49 credits, then 48, 47, 46, etc.

UPDATE: This is what i came up with

_, err := db.Exec("UPDATE dummy SET creds = -1 WHERE email = $1")
if err != nil {
    log.Fatal(err)

}

Instead of working, it fails to do anything.

What am i doing wrong?

Any answers & knowledge into this would be amazing!

Thanks!

Answer 1

As posted by @seva

I believe the question is about returning consistent results. It's not about go then. It's about database. Search "optimistic locking" vs "pessimistic locking", "select for update" etc. Here is also a basic (but good) introduction to working with databases in go (if you need it): go-database-sql.org/index.html