How to get data from database table through GET web API call ? is there a way ? please suggest

A WebAPI call would only be needed if you want to expose Appian data to an external system. If this is not your use case, then Stuart has highlighted how it can be achieved but would strongly suggest you read up on some content about Appian and what functions it can offer. 

https://docs.appian.com/suite/help/19.2/Query_Recipes.html is page all about the query types Appian can offer but it sounds like you might be better off going through some of the tutorials first 

https://docs.appian.com/suite/help/19.2/Tutorials.html

A WebAPI call would only be needed if you want to expose Appian data to an external system. If this is not your use case, then Stuart has highlighted how it can be achieved but would strongly suggest you read up on some content about Appian and what functions it can offer. 

https://docs.appian.com/suite/help/19.2/Query_Recipes.html is page all about the query types Appian can offer but it sounds like you might be better off going through some of the tutorials first 

https://docs.appian.com/suite/help/19.2/Tutorials.html