why we write api test cases code example

Example 1: have you done api testing

I have worked on API testing in my project and I used
POSTMAN for manually testing and REST ASSURED
java LIBRARY for automation.
I used JDBC and it is a Java-based data access technology
used for Java database connectivity. It provides classes and
interfaces to connect or communicate Java application with
database.
JDBC API is a Java API that can access any kind of data
stored in a Relational Database. It enables Java programs to
execute SQL statements.

Example 2: what to test in api

Basicall we are checking response body
to verify if request matches with response.
After that we are writing SQL query
to double check if database matches.
We can say happy path if all pass