SQL is not a programming language. It's a query language (Structured Query Language). You use it to either query a database to find data that fits a set of criteria, or manipulate that data (adding more data, deleting data, or modifying data). SQL is nice because different databases typically respond to basic SQL queries in the same way. More advanced functionality may have differences in syntax between databases, though.
For Spring Boot projects you would typically run your unit tests against an in memory database so it's important that the business logic is not in the database and that the code is database agnostic. The Java 8 API, for example, provides excellent data processing features like SUM, grouping etc. Putting business logic in the database is a retrograde step and should be avoided if possible.
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