✅ Common Table Expressions in SQL 👇

What is a common table expression (CTE)?

A common table expression or CTE is a temporary result set (or a virtual table). It temporarily saves the output of a query to write other queries that reference it.

CTEs are short lived.

CTEs are virtual tables which are created during the execution of a query, used by the query, and eliminated after query execution.

CTE results are not stored anywhere. Think of a CTE as being a View that only lasts for the duration of the query.

Syntax of CTE

CTE starts with a WITHclause, therefore sometimes, they are also called WITH queries.

Two or more CTEs can be added with comma-separation and given unique names (my_cte1, my_cte2) as shown below.

Syntax of a CTE. Image by Author.

CTE can be can referenced within a SELECT, INSERT, UPDATE, or DELETE statement.

Two types of CTE

  1. Nonrecursive CTE — A query for other queries to reference.
  2. Recursive CTE — A query that references itself. It can be considered as a loop.

In the below example, CTE query is being used by other query and hence it is a non-recursive.

A non-recursive CTE is basically a query-local VIEW.

An example of a non-recursive CTE.

In fact, by default CTE is referred to ‘non-recursive’ only until specified with Recursive after WITHclause.

In the below example, CTE uses itself as function 5 times to generate numbers from 1 to 5. This is similar to loops in a programming languages.

In MySQL and PostgreSQL, the RECURSIVE keyword is required.

An example to Recursive CTE to generate the numbers from 1 to 5.

When to Use CTEs

Let me take a sample dataset from Tihomir Babic’s blog for the purpose of the illustration.

Sample Dataset

The university with the following three tables:

  • students
  • subjects
  • exams

The table students has the following columns:

  • id: the ID of the student, a primary key
  • first_name: the student’s first name
  • last_name: the student’s last name

The next table is the subjects table containing the data:

  • id: the ID of the subject, a primary key
  • subject_name: the name of the subject

The third table is the exams table that stores the following data:

  • id: the ID of the exam given
  • exam_date: the date when the exam was given
  • subject_id: the ID of the subject, a foreign key from the table subjects
  • student_id: the ID of the student who took the exam, a foreign key from the table students

Task

To calculate the average minimum grade and the average maximum grade by subject.

Option 1— with functions

Above code will result into an error because SQL doesn’t allow constructions such as AVG (MIN (e.grade)).

Error Message - "Cannot perform an aggregate function on an expression containing an aggregate or a subquery."

Solution 2 — with CTE

CTE vs. Sub-queries

Both CTEs and subqueries allow you to write a query, and then write another query that references the first query.

There are following two advantages of using a CTE instead of a subquery.

1. Multiple References

Once a CTE is defined, you can reference it by name multiple times within the SELECT queries that follow:

Multiple References of a CTE. Image by Author.

With a subquery, you would need to write out the full subquery each time.

2. Multiple Tables

CTE syntax is more readable when working with multiple tables because you can list all the CTEs up front:

Readability with a CTE in Multiple Tables Scenario. Image by Author.

With subqueries, the subqueries would be scattered throughout the overall query.

Thank you for reading this article. If there is anything that is unclear or I’ve made some mistakes in the above feel free to leave a comment. 🙌 🙌

I am certain that you are leaving with a better understanding. Before you leave, please give it a 👏 or leave a comment.

I would highly recommend further reading of following references that I have used in this article —

  1. MySQL Documentation
  2. Tihomir Babic’s blog

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