I have recently started using Node-Red.
I want to understand how could I send data from my MySQL database to postman using Node-Red.
I was advised to use GET option in http in node to send data to postman.
I not sure how it works.
As Mr Backman says, Postman is simply a tool for issuing API calls and seeing the responses. I stopped using it in favour of an extension for VScode that lets me create a suitable file, click to issue commands and get responses back, all inside VScode which is very convenient.
The simplest way to create an API with Node-RED is to use a combination of the http-in and http-out nodes. The -in node defines the API endpoint (URL and Type - e.g. GET/PUT/...). The -out node provides the response back to the requestor. In between, you can add other nodes that formulate the required output.
Database connections go over tcp and ports, not over url's.
So if you know the host (ip/fqdn) your database is running on and it is mysql, it is port 3306. For this you will need to have something like the mysql node installed.
Node-RED provides the URL - you define it when you use the http-in node.
You then need to write a flow that fits between the http-in and http-out nodes that queries your db, returning the results to the msg.payload that you then pass to the http-out node.