Logical query processing order

This lesson uses DDI_SQL_Training dataset version 1.0.0 on SQL Server 2019 or later. The clauses are written SELECT, FROM, WHERE, ORDER BY, yet it helps to reason about them in a different order. That explains why some names are available in one clause but not another.

Run the example

SELECT CustomerID, City AS CustomerCity
FROM dbo.Customers
WHERE City = N'London'
ORDER BY CustomerCity, CustomerID;

Read it step by step

1. FROM dbo.Customers supplies the 20 source rows. WHERE City = N'London' then keeps only the five matching rows. N marks a Unicode string literal in SQL Server.
2. SELECT CustomerID, City AS CustomerCity chooses two output columns and creates the CustomerCity heading.
3. ORDER BY CustomerCity, CustomerID sorts the five output rows. CustomerID breaks ties between customers in the same city.

Check the result

A useful logical sequence is FROM, WHERE, SELECT, ORDER BY, then OFFSET/FETCH. This is a reasoning model for the result, not a claim about the physical steps SQL Server chooses to execute. Because WHERE is logically earlier than SELECT, it uses City rather than the CustomerCity output alias.

Practice

Why can ORDER BY use CustomerCity here, while WHERE uses City? How many London customers are returned?

SELECT COUNT(*) AS LondonCustomers
FROM dbo.Customers
WHERE City = N'London';