SQL Inception with Subqueries

SQL Inception with Subqueries

Have you ever wanted to know how to write T-SQL? This new series covers how to go from having little to no knowledge of T-SQL to a PRO in no time! Austin covers the use of subqueries as a means to filter down data in a temporary result set. 

"Inception" with SQL subqueries refers to the practice of nesting one or more subqueries within a main query. Subqueries, also known as inner queries or nested queries, can be used in various parts of a SQL statement, including the SELECT, FROM, WHERE, and HAVING clauses. They can return a single value, a row of values, or a table of results, depending on the context in which they're used.

Here is a simple example of a SQL query with a subquery in the WHERE clause:

SELECT FirstName, LastName, DepartmentID
FROM Employees
WHERE DepartmentID IN 
  (SELECT DepartmentID 
  FROM Departments 
  WHERE DepartmentName = 'Sales');

In this example, the subquery (the part inside the parentheses) first selects the ID of the department named 'Sales'. The main query then selects the first and last names of all employees who work in that department.

You can have multiple levels of subqueries if needed, hence the "inception" reference. However, keep in mind that too many nested subqueries can make your SQL statements difficult to read and understand, and it can also affect performance. Using joins or temporary tables might be more efficient alternatives in some cases.

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