SQL - Common Table Expression (CTE)

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A Common Table Expression (CTE) can make it easier to manage and write complex queries by making them more readable and simple, like database views and derived tables. We can reuse or rewrite the query by breaking down the complex queries into simple blocks.

The SQL Common Table Expression

The WITH clause in MySQL is used to specify a Common Table Expression.

A Common Table Expression (CTE) in SQL is a one-time result set, i.e. it is a temporary table that exists only during the execution of a single query. It allows us to work with data specifically within that query, such as using it in SELECT, UPDATE, INSERT, DELETE, CREATE, VIEW, OR MERGE statements.

CTE is temporary because it cannot be stored anywhere for later use; once the query is executed, it is lost.

The MySQL WITH Clause

To specify common table expressions, we use WITH clause that consists of one or more comma-separated subclauses. Within each subclause, we can present a subquery that produces a result set and assigns a name to this subquery.

You cannot use the WITH clause in MySQL versions before 8.0.

Syntax

Following is the syntax to create a CTE using WITH clause −

WITH CTE_NAME (column_name) AS (query)
SELECT * FROM CTE_NAME;

Where,

• CTE_NAME − It is the name assigned to the CTE.
• column_name − It is the column names for the CTE, which can be useful for improving query readability.
• query − It defines the CTE and it can be any valid SQL query.
• After defining the CTE, you can reference it in subsequent queries within the same session.

Example

Assume we have created a table named CUSTOMERS in MySQL database using CREATE TABLE statement as shown below −

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

Now, we are inserting some records into the above created table −

INSERT INTO CUSTOMERS VALUES 
(1, 'Ramesh', 32, 'Ahmedabad', 2000.00 ),
(2, 'Khilan', 25, 'Delhi', 1500.00 ),
(3, 'Kaushik', 23, 'Kota', 2000.00 ),
(4, 'Chaitali', 25, 'Mumbai', 6500.00 ),
(5, 'Hardik', 27, 'Bhopal', 8500.00 ),
(6, 'Komal', 22, 'Hyderabad', 4500.00 ),
(7, 'Muffy', 24, 'Indore', 10000.00 );

The table created is as shown below −

Here, we are creating a Common Table Expression (CTE) named CUSTOMER_AGE that selects all customers with an age of 23. We are then retrieving the ID, NAME, and AGE of these customers from the CTE.

WITH CUSTOMER_AGE AS (
SELECT * FROM customers WHERE AGE = 23)
SELECT ID, NAME, AGE FROM CUSTOMER_AGE;

Output

Following is the output of the above query −

CTE from Multiple Tables

We can also create a Common Table Expression (CTE) that combines data from multiple tables by using JOIN operations within the CTE's subquery. To do this, we need to use the comma operator to separate each CTE definition, effectively merging them into a single statement.

Syntax

Following is the basic syntax for multiple Common Table Expression (CTE) −

WITH
   CTE_NAME1 (column_name) AS (query),
   CTE_NAME2 (column_name) AS (query)
SELECT * FROM CTE_NAME1
UNION ALL
SELECT * FROM CTE_NAME2;

We can use multiple Common Table Expressions (CTEs) with various SQL operations, such as UNION, UNION ALL, JOIN, INTERSECT, or EXCEPT.

Example

In here, we are defining two CTEs namely 'CUSTOMERS_IN_DELHI' and 'CUSTOMERS_IN_MUMBAI' to segregate customers based on their addresses in Delhi and Mumbai. Then, we are using the UNION ALL operator to combine the results from both CTEs into a single result set, retrieving customer information from both cities.

WITH
CUSTOMERS_IN_DELHI AS (
   SELECT * FROM CUSTOMERS WHERE ADDRESS = 'Delhi'),
CUSTOMERS_IN_MUMBAI AS (
   SELECT * FROM CUSTOMERS WHERE ADDRESS = 'Mumbai')
SELECT ID, NAME, ADDRESS FROM CUSTOMERS_IN_DELHI
UNION ALL
SELECT ID, NAME, ADDRESS FROM CUSTOMERS_IN_MUMBAI;

Output

Output of the above query is as shown below −

Recursive CTE

A common table expression is a query that keeps referring back to its own result in a loop repeatedly until it returns an empty result.

A recursive query continually iterates across a subset of the data during its execution, and defines itself in a self-referencing manner. This self-referencing mechanism allows it to repeatedly process and expand its results until a stopping condition is met.

To make a CTE recursive, it must include a UNION ALL statement and provide a second definition of the query that utilizes the CTE itself. This allows the CTE to repeatedly reference to its own results, creating a recursive behaviour in the query.

Example

Now, we are using a recursive CTE named recursive_cust to retrieve data from the 'CUSTOMERS' table created above. Initially, we are selecting customers with salaries above 3000 and then recursively appending customers older than 25 to the result set using the UNION ALL operator −

WITH recursive_cust (ID, NAME, ADDRESS, AGE) AS (
   SELECT ID, NAME, ADDRESS, AGE
   FROM CUSTOMERS
   WHERE SALARY > 3000
   UNION ALL
   SELECT ID, NAME, ADDRESS, AGE
   FROM CUSTOMERS
   WHERE AGE > 25
)
SELECT * FROM recursive_cust;

Output

When the above query is executed, all data from the customers table whose age is greater than 25 or salary is greater than 3000 will be displayed recursively as shown below −

Example

In the following query, we are using a recursive CTE named Numbers to generate and display numbers from 1 to 5. The recursive part continually adds 1 to the previous value until it reaches 5, creating a sequence −

WITH RECURSIVE Numbers AS (
  SELECT 1 AS N
  UNION ALL
  SELECT N + 1 FROM Numbers WHERE N < 5
)
SELECT n FROM Numbers;

Output

After executing the above query, we get the following output −

Advantages of CTE

Following are the advantages of the CTE −

• CTE makes the code maintenance easier.

• It increases the readability of the code.

• It increases the performance of the query.

• CTE allows for the simple implementation of recursive queries.

Disadvantages of CTE

Following are the disadvantages of the CTE −

• CTE can only be referenced once by the recursive member.

• We cannot use the table variables and CTEs as parameters in a stored procedure.

• A CTE can be used in place of a view, but a CTE cannot be nested while views can.