Auto increment adds one to the value on every row inserted, I always stick 'NULL' in there. For instance, you have these rows

ID Name
1 - Lazy
2 - Foo
3 - Bar

All you have to tell the database to assign is the name, it will take care of the ID itself.

If you delete 3 and insert another row, it will look like this
ID Name
1 - Lazy
2 - Foo
4 - Monkey

It will always add one, not from the row count or the last value, it keeps a value elsewhere. All your primary keys should be auto_increment unless you have a good reason otherwise (I've yet to find one)

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