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Editor to share with you what is the use of CTE in SQL. I hope you will gain a lot after reading this article. Let's discuss it together.

CTE, which represents a common table expression, is a temporarily named result set that always returns a result set. It was introduced by standard SQL to simplify SQL queries. The following article will introduce you to CTE (Common Table expression). I hope it will be helpful to you.

What is CTE?

A common table expression (CTE) can be thought of as a temporary result set defined within the execution scope of a single SELECT,INSERT,UPDATE,DELETE or CREATE VIEW statement. CTE is similar to a derived table in that it is not stored as an object and persists only during the query. Unlike derived tables, CTE can be self-referenced and can be referenced multiple times in the same query. [recommended for related video tutorials: MySQL tutorial]

The structure of CTE

CTE consists of an expression name that represents CTE, an AS keyword, and a SELECT statement. After you define a CTE, you can reference it like a table or view in a SELECT,INSERT,UPDATE or DELETE statement. CTE can also be used as part of its definition SELECT statement in a CREATE VIEW statement.

The basic syntax structure of CTE is:

WITH Expression_Name [(ColumnName [1m. N])] AS (CTE query definition)

Note: we can define CTE by adding a WITH clause directly before the SELECT,INSERT,UPDATE,DELETE or MERGE statement. The WITH clause can contain one or more comma-separated CTE.

The declaration to run CTE is:

SELECT FROM expression_name

Type of CTE

There are two types of CTE: recursive and non-recursive.

Recursive CTE: is a common table expression that references itself.

Non-recursive CTE, as the name implies, does not use recursion; they do not refer to themselves.

Benefits of using CTE

○ readability: CTE improves readability. Instead of centralizing all query logic into one large query, create several CTE that will be combined at the end of the statement. This allows you to get the blocks you need and combine them into the final SELECT.

○ alternative view: you can replace the view with CTE. It is convenient if you do not have permission to create a view object, or if you do not want to create a view object because it is used only in this one query.

○ recursion: using CTE creates a recursive query, that is, you can invoke your own query. This is convenient when you need to deal with hierarchical data such as organization charts.

○ restrictions: overcome the limitations of SELECT statements, such as referencing itself (recursion) or using non-deterministic functions to execute GROUP BY.

○ ranking: whenever you want to use ranking functions, such as ROW_NUMBER (), RANK (), NTILE (), etc.

After reading this article, I believe you have a certain understanding of the use of CTE in SQL, want to know more about it, welcome to follow the industry information channel, thank you for reading!

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