Transact SQL primarily refers to the procedural language used with the Sybase and Microsoft SQL Server DBMS products. 
The language supports standard sql language expressions, variables, flow-of-control expressions, session directives, and server directives.  

Typically one refers to "Transact SQL" when talking about Sybase, and "T-SQL" for MSSQL.  This convention holds up in the documentation of both of these respective products.  The dialects still look almost identical, but have diverged slightly.
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CategoryQueryLanguage