What are SQL CTEs ?

SQL Common Table Expressions (CTEs) are a powerful tool for writing and organizing complex SQL queries. They are used to simplify the logic and structure of complex SQL queries, making them easier to understand and maintain. In this blog post, we will take a closer look at what SQL CTEs are, how they can be used, and how they differ from temp tables.

A SQL CTE is a named temporary result set that can be used within a SELECT, INSERT, UPDATE, or DELETE statement. It is essentially a named subquery that can be referenced within the main query. CTEs can be used to simplify complex queries by breaking them down into smaller, more manageable pieces. They can also be used to simplify complex recursive queries, as well as to improve query performance by reducing the amount of data that needs to be processed.

Here are a few examples of how SQL CTEs can be used:

Simplifying complex queries: Consider a query that calculates the total sales for each product category for a given time period. The query can be simplified using a CTE to break down the calculation into smaller, more manageable pieces.

WITH SalesCTE AS
(
    SELECT 
        Category, 
        SUM(SalesAmount) AS TotalSales
    FROM 
        Orders
    WHERE 
        OrderDate BETWEEN '2021-01-01' AND '2021-12-31'
    GROUP BY 
        Category
)
SELECT 
    Category, 
    TotalSales
FROM 
    SalesCTE

Improving query performance: SQL CTEs can also be used to improve query performance. For example, if a query is executed multiple times with the same parameters, a CTE can be used to store the result of the first execution, which can then be reused in subsequent executions.

WITH CustomerOrdersCTE AS
(
    SELECT 
        CustomerID, 
        OrderID, 
        OrderDate
    FROM 
        Orders
    WHERE 
        OrderDate BETWEEN '2021-01-01' AND '2021-12-31'
)
SELECT 
    CustomerID, 
    COUNT(OrderID) AS TotalOrders
FROM 
    CustomerOrdersCTE
GROUP BY 
    CustomerID

Recursive Queries: CTEs can be used to simplify recursive queries, which are queries that involve data relationships that repeat multiple times. For example, consider a query that calculates the number of employees who report to each manager in an organization.

WITH EmployeeCTE (EmployeeID, ManagerID, Level)
AS
(
    SELECT 
        EmployeeID, 
        ManagerID, 
        0 AS Level
    FROM 
        Employees
    WHERE 
        ManagerID IS NULL
    UNION ALL
    SELECT 
        e.EmployeeID, 
        e.ManagerID, 
        cte.Level + 1
    FROM 
        Employees e
    JOIN 
        EmployeeCTE cte ON e.ManagerID = cte.EmployeeID
)
SELECT 
    ManagerID, 
    COUNT(EmployeeID) AS NumberOfEmployees
FROM 
    EmployeeCTE
GROUP BY 
    ManagerID

SQL CTEs are supported by most major relational database management systems, including Microsoft SQL Server, Oracle, PostgreSQL, and MySQL 8.0 and higher.

SQL CTEs differ from temp tables in several important ways. Temp tables are physical tables that are created in the database and persist until they are dropped. CTEs, on the other hand, are temporary result sets that exist only for the duration of a single query. Additionally, temp tables can be used in multiple queries, whereas CTEs can only be used within a single query.

In conclusion, SQL Common Table Expressions (CTEs) are a useful tool for organizing and simplifying complex SQL queries. By breaking down complex queries into smaller, more manageable pieces, CTEs can make it easier to understand and maintain complex SQL code. They are supported by most major relational database management systems, and offer a more flexible and lightweight alternative to temp tables.