Why Did Microsoft Create its Own SQL Extension (T-SQL)?

Question

What are the reasons behind Microsoft implementing its own SQL extension as Transact SQL (T-SQL)? What are its advantages over the normal SQL?

Answer 1

Transact-SQL (T-SQL) is Microsoft's and Sybase's proprietary extension to SQL. Microsoft's implementation ships in the Microsoft SQL Server product. Sybase uses the language in its Adaptive Server Enterprise, the successor to Sybase SQL Server. Transact-SQL enhances SQL with these additional features:

• Control-of-flow language
  
• Local variables
  
• Various support functions for string processing, date processing, mathematics, etc.
  
• Improvements[citation needed] to DELETE and UPDATE statements

http://en.wikipedia.org/wiki/Transact-SQL

Wiki connects you to much more expanding information and details.

Answer 2

Everybody extends SQL.

SQL isn't procedural, it's declarative. You describe what you want, and it figures out how to retrieve it using whatever indexes or hashes or whatnot is available.

But sometimes, that's not sufficient. T-SQL gives syntax to run procedural code inside of your queries. This lets you do control structures (begin-end, if-then-else), iteration and move values between local variables, temporary tables and other sources.