Deferral and database lookup of player (entirely untested, educational purposes only)

server.lua

AddEventHandler('playerConnecting', function(playerName, setKickReason, deferrals)
    -- First, a local copy of source, so it doesn't get lost while deferring.
    local source = source
    
    -- Actually defer the connection
    deferrals.defer()
    -- Wait a tick, so the deferral happens.
    Citizen.Wait(0)

    -- Tell the client what we're doing, in case it stalls at this point, somehow.
    deferrals.update('Updating player name...')

    -- Grab the identifier we need.
    local identifier = GetPlayerIdentifierByType(source, 'license')

    -- Query the database
    MySQL.Async.insert('INSERT INTO Data (identifier, spelernaam) VALUES (@identifier, @spelernaam) ON DUPLICATE KEY UPDATE spelernaam = @spelernaam', {
        ['@identifier'] = identifier,
        ['@spelernaam'] = playerName
    }, function(rowsAffected)
        if rowsAffected > 0 then
            --[[ This makes no sense, as the player's chat resource isn't running yet.
            TriggerClientEvent('chat:addMessage', source, {
                args = {'Your data has been processed.'}
            })
            --]]
            deferrals.done() -- Admit the player.
        else
            -- Failure, so we deny the player.
            deferrals.done('Could not update player name, or whatever')
            print('Error processing data for ' .. playerName)
        end
    end)
end)