There are several tools, platforms, SaaS services, and languages that can help you quickly build a flexible API on top of a SQL Server database. Here are some popular options:

1. ASP.NET Core:

   • ASP.NET Core is a cross-platform framework developed by Microsoft for building web applications and APIs.
   • It provides a powerful set of tools and libraries for creating REST APIs and supports integration with SQL Server databases using Entity Framework Core.
   • Language: C#

2. Node.js with Express.js:

   • Node.js is a JavaScript runtime environment that allows you to build server-side applications.
   • Express.js is a popular web application framework for Node.js that simplifies the process of creating APIs.
   • You can use libraries like Sequelize or Knex.js to integrate with SQL Server databases.
   • Language: TypeScript, JavaScript

3. Django (Python):

   • Django is a high-level Python web framework that follows the Model-View-Controller (MVC) architectural pattern.
   • It provides an Object-Relational Mapping (ORM) layer that abstracts database operations and supports SQL Server databases.
   • Django REST Framework is an extension of Django that simplifies the creation of RESTful APIs.
   • Language: Python

4. Ruby on Rails:

   • Ruby on Rails is a web application framework written in the Ruby programming language.
   • It follows the convention over configuration principle and provides a simple and expressive way to build APIs.
   • ActiveRecord, the ORM layer in Rails, supports SQL Server databases through the activerecord-sqlserver-adapter gem.
   • Language: Ruby

5. Hasura (GraphQL):

   • Hasura is an open-source GraphQL engine that sits on top of your database and provides instant GraphQL APIs.
   • It supports SQL Server databases and automatically generates GraphQL schemas and resolvers based on your database schema.
   • You can use Hasura to quickly build GraphQL APIs without writing much code.
   • Language: N/A (Hasura generates the API layer)

6. Prisma (GraphQL):

   • Prisma is an open-source database toolkit that simplifies database access and management.
   • It supports SQL Server databases and provides a type-safe API layer for building GraphQL APIs.
   • Prisma generates the GraphQL schema and resolvers based on your database schema, making it easy to create APIs quickly.
   • Language: TypeScript, JavaScript

7. Loopback (Node.js):

   • Loopback is an open-source Node.js framework for building APIs and microservices.
   • It provides a powerful CLI and a set of abstractions for creating REST APIs quickly.
   • Loopback supports SQL Server databases through the loopback-connector-mssql package.
   • Language: TypeScript, JavaScript

These are just a few examples, and there are many other tools and frameworks available depending on your preferred programming language and API style. Choose the one that aligns with your development skills and project requirements.