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Subquery Tutorial.sql

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+/*
+
+Today's Topic: Subqueries (in the Select, From, and Where Statement)
+
+*/
+
+Select EmployeeID, JobTitle, Salary
+From EmployeeSalary
+
+-- Subquery in Select
+
+Select EmployeeID, Salary, (Select AVG(Salary) From EmployeeSalary) as AllAvgSalary
+From EmployeeSalary
+
+-- How to do it with Partition By
+Select EmployeeID, Salary, AVG(Salary) over () as AllAvgSalary
+From EmployeeSalary
+
+-- Why Group By doesn't work
+Select EmployeeID, Salary, AVG(Salary) as AllAvgSalary
+From EmployeeSalary
+Group By EmployeeID, Salary
+order by EmployeeID
+
+
+-- Subquery in From
+
+Select a.EmployeeID, AllAvgSalary
+From 
+	(Select EmployeeID, Salary, AVG(Salary) over () as AllAvgSalary
+	 From EmployeeSalary) a
+Order by a.EmployeeID
+
+
+-- Subquery in Where
+
+
+Select EmployeeID, JobTitle, Salary
+From EmployeeSalary
+where EmployeeID in (
+	Select EmployeeID 
+	From EmployeeDemographics
+	where Age > 30)