Hopeless Idealist wrote: I would rather type:...but all the companies demand SQL-devs. First of course because companies do not operate to please you. They operate to make money. And that comes from pleasing customers. Both in sales and in follow up support. Additionally they must keep costs low. At the end of the day technologies do not drive company success. Rather it is matter of successfully creating a product(s)/service(s) which the company can maintain and enhance. And of course then selling it in the first place. Large companies almost always have multiple persisted datastores. And I only say 'almost' because I can't be sure that there is not one company out there that only uses one. I do know that Microsoft, Oracle, Netflix, Google and Amazon all use different ones.