Storing random strings without duplicates in a database

I am generating strings to use as IDs, so to avoid repetitions I must currently check in my database if that string has already been assigned and if so, generate another one, what I want is avoid the rework of calculating another string.

I can generate and store all possible strings to assign them more quickly but I am not convinced by this idea

Can you think of something more efficient?

I edit: the requirement demands that I use identifiers of this type, nothing personal;)

The lifetime of each ID is 24 hours