Transact-SQL (T-SQL) is a proprietary extension to the Structured Query Language (SQL) used to interact with relational databases, developed by Microsoft and Sybase. T-SQL expands on the SQL standard to include procedural programming, local variables, various support functions for string processing, date processing, mathematics, etc., and changes to the DELETE and UPDATE statements. T-SQL is central to using Microsoft SQL Server, and all applications that communicate with an instance of SQL Server do so by sending Transact-SQL statements to the server, regardless of the user interface of the application.

T-SQL queries include the SELECT statement, selecting columns, labeling output columns, restricting rows, and modifying a search condition. T-SQL identifiers are used in all databases, servers, and database objects in SQL Server, including tables, constraints, stored procedures, views, columns, and data types. T-SQL identifiers must each have a unique name, are assigned when an object is created, and are used to identify an object.

T-SQL is a procedural language that adds declared variables, transaction control, error and exception handling, and row processing to SQL. It has the same functionality and gives similar results to other languages, but with some added extras and differences that make it unique. T-SQL provides some unique features to the users, such as a BULK INSERT statement that allows users to import a file into the database table or view in a user-specified format, and various support functions for string processing and data processing.

In summary, T-SQL is a proprietary extension to SQL used to interact with relational databases, developed by Microsoft and Sybase. It expands on the SQL standard to include procedural programming, local variables, various support functions, and changes to the DELETE and UPDATE statements. T-SQL is central to using Microsoft SQL Server, and all applications that communicate with an instance of SQL Server do so by sending Transact-SQL statements to the server.