SQL Fundamentals: Logical Query Processing

Data Engineering Academy
4 min readApr 3, 2021

SQL is a declarative English-like language. This means you specify the result set you want returned and it’s up to the SQL query engine to figure out how best to retrieve the result set from the database. This is opposed to a imperative language which also defines the steps to achieve what you want.

Originally SQL was called SEQUEL which stood for “structured English query Language” but had to be renamed to SQL because of a trademark dispute with an airline. Still, the point is that you provide your instructions in an English-like manner. For example, consider the instruction, “Bring me all the cake from the cupboard”. Observe that in the instruction in English, the object comes before the location. A similar query in SQL would take the form of:

Select chocolate from Kitchen.cupboard

Now if you wanted to relay this request to a three year old (promise I don’t do this 😉) or a robot, you would need to define logical order in which to execute the steps. It makes sense that the instructions would be something like “Go into the Kitchen; go to the cupboard, open the door, get the chocolate and bring it to me”. Similarly with SQL, in order for the logical query processor to know where to retrieve the data points, it must first know which table(s) to pull them from. So contrary to the keyed order of the query above, the logical processing order is as follows:

From Kitchen.cupboard Select chocolate

Logical Query Processing Order

If you know the concept of logical query processing well, you will understand many things about the way the language behaves.

Key Order vs Logical Processing Order

Each logical processing step receives a table as an input and produces a table as an output. Let’s illustrate each step with using the below Customer Order table and query:

Customer Order Table
Select CustomerName, sum(OrderAmount) as TotalSpend 
From Customer
Where OrderDate > "01/01/2021"
Group By CustomerName
Having sum(OrderAmount) > 50
Order By TotalSpend

1. From Clause