Do you know the foundation of SQL?
According to the book Querying Microsoft SQL Server 2012 Training Kit on the Microsoft label, T-SQL is a Microsoft flavour of the more standard ANSI and ISO version of SQL which is based on a branch of mathematics called Relational Model.
Relational Model is further broken down into two theories – Set Theory and Predicate Logic.
Set Theory says that interaction with a set is not with the individual elements but with the set of elements on a whole. A set consists of a collection of distinct, definite objects.
Predicate Logic is when an expression evaluates out to true or false.
The combination of Set Theory and Predicate Logic gives you the foundation of SQL.
How is this useful?
Set theory implies that all elements are unique (distinct) and the order does not matter. Therefore, when constructing T-SQL queries, one considers the set of elements as a whole when manipulating data. This means that you are already walking in the right direction. Now you need to decide if you are trying to get results fast for a one off query, or optimising the results for repeat use (as used in business reports). Only deviate from standard relational model when it finally comes to presentation. For example, by adding the ORDER BY clause to sort the data.