My company's product has a big relational database storage(around 20 TB) and we want to move all or some part of our database. Now I am a bit confused regarding which cloud SQL service will be suitable for me. I have read that some cloud storage only provides specific SQLs like MySQL ... which I cannot afford, the reason is that here we have lots of Transact-SQL scripts on our MSSQL server, which periodically updates the data.
So what should be my basic approach to achieve that, How can I compare between various cloud-based relational database storage like Google Cloud SQL, Azure SQL and Amazon RDS?
If your existing SQL server is Microsoft SQL Server then it probably makes the most sense to use a Microsoft SQL server offering in a cloud environment. That will help minimize any sorts of compatibility issues and also minimize the need to learn a new SQL server environment.
Amazon's RDS service lets you choose between various versions of MySQL, Postgres, Oracle, and Microsoft SQL Server. With regards to Microsoft SQL Server, Amazon offers an express edition, as well as SE and EE versions. As far as licensing goes, they offer both "bring your own" as well as pay-by-usage licenses. More information on the license details can be found in their RDS documentation. They also have a usage calculator that lets you estimate what your monthly costs will be based on the license you use, the size of the instance, etc.
Since Azure SQL is a Microsoft product I would hope that it's compatible with Microsoft SQL Server but I have never used it so I honestly have no idea if there are any compatibility issues. Microsoft also has some basic pricing information available for Azure SQL so you can work out an estimate of your costs if you go with them.
If you're concerned about any scripts working properly on one of the cloud platforms then I'd suggest creating a small test database that you can thoroughly test those scripts with. Upload that test database to the different clouds and test the scripts there to ensure they function properly. Amazon offers a very light-weight RDS instance for free so you can do basic testing without it costing you anything. Perhaps Azure offers something similar. Even if they don't or the free RDS instance isn't powerful enough for your testing needs, spinning up an instance for a day or two to run these sorts of tests shouldn't cost you very much.