When should I use a NoSQL database instead of a relational database? Is it okay to use both on the same site?

Relational databases enforces ACID . So, you will have schema based transaction oriented data stores. It's proven and suitable for 99% of the real world applications.

You can practically do anything with relational databases.

NoSQL solutions are usually meant to solve a problem that relational databases are either not well suited for, too expansive to use (Oracle) or require you to implement something that breaks the relational nature of your db anyway.

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