SQL is a detail. Knowledge of a detail should not spread.

As SQL is used in more and more places in your code your code becomes more and more dependent on it. 

As you learn more and more SQL tricks you solve more and more problems using SQL. This means that switching to another API to persist involves more than just translating. You have to solve problems you didn't realize you had.

You run into this even switching between Databases. One offers fancy whizzbang feature 5 so you use it in a number of places only to find out fancy whizzbang feature 5 is proprietary and now you have a licensing issue that's going to cost a lot of money. So you do a lot of work digging up everywhere you used feature 5 and solving the problem on your own only to find out later you're also using whizzbang feature 3.

One of the things that makes Java so portable is that certain features of the CPU just aren't available. If they were available I'd use them. And suddenly there are CPU's that my Java code won't work on because they don't have those features. It's the same with database features. 

It's all to easy to sacrifice your independence without realizing it. SQL is a choice not a given. If you make the decision to use SQL then make it in one place. Make it in a way that can be unmade.

The fact that SQL has security issues and that we're moving to persistent memory models doesn't mean SQL is doomed. It just drives home the point that it's a choice. If you want to preserve the right to make that choice you have to do the work. 

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It may be worth noting that the database movement of the 80's and Uncle Bob have a rather nasty history. He had all his problems solved with a flat file system when management forced a database admin into his life. This event pushed him into his stellar consulting career. (He tells this story in one of his early clean books, forget which) He knows how to solve problems without DB's and has little patience for those that act like using them is a given.

He also tells a story about putting off adding a DB to an application until the last minute when a customer demanded it, and added it in a day as an optional feature. My guess is he see's the way most of us use DB's as an addiction. He's trying to show us how to kick the habit.