to CTEs in SQL:
Structured Query Language (SQL) is a standard language used by database management systems (DBMS) to retrieve and modify data stored in relational databases. An essential part of using SQL is understanding how to use Common Table Expressions (CTEs) to improve code readability, maintainability, and efficiency.
In this article, we will explain what CTEs are, how they are used, and provide an example of their usage in a SQL query. Definition and Basic Syntax:
A Common Table Expression (CTE) is a temporary named result set that you can reference within a SELECT, DELETE, INSERT, or UPDATE statement.
CTEs provide a modern SQL concept that can help developers write more readable code by breaking down highly complex queries into smaller, manageable chunks. The basic syntax for creating a CTE is:
WITH
cte_name (column1, column2, columnN) AS
(
— CTE query definition
)
Using CTEs for Improved Code Readability and Maintainability:
Code maintainability, readability, and reproducibility are essential factors when writing SQL queries for large and complex databases. CTEs allow developers to create named result sets within a single SQL statement, reducing the need to write multiple nested queries for a more straightforward and intuitive understanding of the code structure.
By breaking down a complex query into smaller sections, developers can easily identify and troubleshoot issues, and make modifications without affecting other parts of the code. Additionally, CTEs provide a simple way to reuse a result set in more complex queries, reducing the amount of code repetition throughout the project.
Example of CTE Usage:
Now that we have discussed the definition and importance of CTEs, let’s take a look at an example of their use in a SQL query. Suppose you have a database that stores job offers and employee occupation details in different tables.
You need to list all the employees’ names, their respective occupation, and the average salary of each occupation. You can do this by joining the two tables, grouping the employees by occupation, and calculating the average salary for each group.
Suppose the job offers table has the following columns:
– Job_ID
– Job_Offer_Title
– Occupation_ID
And the employee_occupation table has the following columns:
– Employee_ID
– First_Name
– Last_Name
– Occupation_ID
– Salary
To create the desired output, you will need to use a CTE to first calculate the average salary of each occupation.
WITH avg_salary_cte AS
(
SELECT
Occupation_ID,
AVG(Salary) AS avg_salary
FROM
employee_occupation
GROUP BY
Occupation_ID
)
Next, you can use a LEFT JOIN to combine the avg_salary_cte and employee_occupation tables on the Occupation_ID column, allowing you to pull the salary and occupation details for each employee.
SELECT
eo.first_name,
eo.last_name,
eo.Occupation_ID,
a.avg_salary
FROM
employee_occupation eo
LEFT JOIN avg_salary_cte a on eo.Occupation_ID = a.Occupation_ID
The result will output a table that lists the employees’ names and their respective occupation, along with the average salary for each occupation:
| first_name | last_name | Occupation_ID | avg_salary |
|————|———–|————–|————|
| John | Doe | 1 | $50,000 |
| Jane | Smith | 2 | $60,000 |
| Bob | Johnson | 1 | $50,000 |
| Mary | Davis | 3 | $70,000 |
By using CTEs, you can create a temporary named result set that contains the average salary for each occupation, making it easier to join and group data from multiple tables. This example illustrates how CTEs can be used to improve code readability and maintainability, which is crucial when working with large and complex databases.
Conclusion:
In this article, we have introduced CTEs in SQL, defined their basic syntax, and explained how they can be used to improve code readability and maintainability. We also provided an example of their use in a SQL query to illustrate how CTEs can be used to join and group data from multiple tables.
Whether you are new to SQL or an experienced developer, CTEs are an essential concept to understand, as they can help you write better, more efficient queries for your database. 3) CTEs vs.
Subqueries
When it comes to querying relational databases in SQL, developers have two primary options for breaking down complex inquiries: Common Table Expressions (CTEs) and subqueries. Subqueries are essentially queries that are nested inside another query, while CTEs provide named, temporary result sets that can be referenced repeatedly within a single SQL statement.
So, what are the benefits of using a CTE instead of a subquery? Benefits of Using CTEs Over Subqueries:
One significant benefit of using CTEs over subqueries is their superior code readability.
As queries become longer and more complicated, subqueries can become challenging to read and understand, making them time-consuming and error-prone to work with. In contrast, CTEs allow developers to break down complex queries into smaller and more manageable pieces, which can significantly improve the readability of long SQL statements.
Another advantage of using CTEs is that they can be more efficient than subqueries. With subqueries, the DBMS must execute the inner query before moving on to the outer query, meaning that the same data may be queried and retrieved multiple times.
On the other hand, CTEs can be executed just once, allowing for more efficient querying of databases and reduced latency. Example of Using CTEs vs.
Subqueries:
Here is an example of how you can use a CTE instead of subquery and get the same output in a simpler, more manageable way. Suppose you have a database table called “Employees” with columns “employee_id,” “first_name,” and “last_name.” You want to retrieve a list of all employees who were hired before a specific date and have the same last name as a particular employee.
Here is how you can use a subquery to create the output:
SELECT *
FROM Employees
WHERE hire_date < (
SELECT hire_date
FROM Employees
WHERE last_name = ‘Doe’
) AND last_name = ‘Doe’;
And here is how you can achieve the same output using a CTE:
WITH named_employee AS (
SELECT hire_date
FROM Employees
WHERE last_name = ‘Doe’
)
SELECT *
FROM Employees
WHERE hire_date < (
SELECT hire_date
FROM named_employee
) AND last_name = ‘Doe’;
As you can see, the CTE code is easier to read and understand than the subquery, making it a more desirable option when dealing with complicated queries.
4) Multiple CTEs in a Single SQL Statement
Sometimes, creating a SQL query requires several CTEs to be named and used within a single SQL statement. This situation is common when handling long and complicated queries, nested queries, and hierarchical data.
So how do you use multiple CTEs in a single SQL statement? Need for Multiple CTEs:
The need for multiple CTEs arises when you have a long and complicated SQL query that requires different named temporary result sets to perform multiple tasks quickly.
Using multiple CTEs in a single SQL statement can make a query more manageable by defining a hierarchy or grouping of results. Example of Using Multiple CTEs:
Suppose you have a hierarchical data structure that requires the use of multiple named result sets to retrieve the necessary data.
You can use nested CTEs, where each CTE refers to another CTE in the same query based on its name. You can use a recursive CTE to traverse a tree-like structure continuously, while you can use a non-recursive CTE to resolve dependencies among the named result sets.
Here is an example of using multiple CTEs to find all the managers for a given employee ID in a company’s employee tree:
WITH EmployeeCTE AS (
SELECT EmployeeID, ManagerID, FirstName, LastName
FROM Employees
WHERE EmployeeID = @EmployeeID
), ManagerCTE AS (
SELECT EmployeeID, ManagerID, FirstName, LastName
FROM Employees
WHERE ManagerID IN (
SELECT EmployeeID FROM EmployeeCTE)
), SubManagerCTE AS (
SELECT EmployeeID, ManagerID, FirstName, LastName
FROM Employees
WHERE ManagerID IN (
SELECT EmployeeID FROM ManagerCTE)
)
SELECT * FROM ManagerCTE
UNION ALL
SELECT * FROM SubManagerCTE;
In the above example, the first CTE EmployeeCTE retrieves the information for a specific employee ID. Next, the ManagerCTE selects all employees who have the first CTE’s employee ID as their manager ID.
The next CTE, SubManagerCTE, uses the ManagerCTE’s result set to select all employees who report to those who have the first CTE’s employee ID as their manager ID. Using multiple CTEs allows you to achieve the desired results in an organized and structured way, making it easier to read and understand the code.
The use of CTEs for complex queries can increase code maintainability and reproducibility. Conclusion:
CTEs are a valuable tool for working with relational databases in SQL.
By using CTEs instead of subqueries, you can achieve greater code readability and more efficient querying. When dealing with long and complicated queries, multiple CTEs can help developers structure their queries in a more organized, hierarchical way.
By breaking down queries into smaller, more manageable parts, you can achieve faster, more accurate results while reducing the likelihood of errors. 5) Using CTEs Instead of Creating Tables:
Creating tables is a valid practice in SQL, but it comes with unnecessary metadata that can make maintaining and updating data more challenging.
For instance, if you create a table to hold data that you will use for a single query or purpose, you end up having to keep that table updated, even if you only needed its data for a short period. This can lead to cluttered databases and harm performance.
Luckily, developers can use CTEs as an alternative to tables. Here’s what you need to know about using CTEs instead of creating tables.
Benefits of Using CTEs Instead of Tables:
One of the significant benefits of using CTEs over creating tables is that CTEs offer a temporary named result set that can be used within the same SQL query. This means that you don’t have to create a table to store data because CTEs are temporary and available only in the current query.
This can help reduce the amount of clutter in the database and also improve the performance. Another advantage of using CTEs instead of tables is the ability to join data from various tables in the database without creating real tables.
CTEs are intuitive, and their names make it easy to understand what data is being pulled and how it’s being used. Example of Using CTEs Instead of Creating Tables:
Consider this scenario: you need to query the database to retrieve a list of all employees who have worked with a particular client in the last month.
Here’s how you can use a CTE instead of creating a table:
WITH LastMonthOrders AS (
SELECT * FROM Orders
WHERE OrderDate >= DATEADD(month, -1, GETDATE())
),
ClientEmployees AS (
SELECT E.EmployeeID, E.FirstName, E.LastName, Orders.OrderID
FROM Employees E
JOIN Orders ON E.EmployeeID = Orders.EmployeeID
JOIN Clients C ON Orders.ClientID = C.ClientID
WHERE C.Name = ‘Client X’
)
SELECT DISTINCT EmployeeID, FirstName, LastName
FROM ClientEmployees
JOIN LastMonthOrders ON ClientEmployees.OrderID = LastMonthOrders.OrderID;
In this example, we use two CTEs to accomplish the task instead of creating a table. The first CTE, LastMonthOrders, filters out orders from the last month.
The second CTE, ClientEmployees, joins the Employees, Orders, and Clients tables and returns only the relevant data. The final SQL query then joins the two CTEs to automatically pull the employees that are relevant to our query.
This example highlights how CTEs can significantly improve code readability and maintenance without creating tables unnecessarily. 6) Conclusion and Recommendations for Learning:
CTEs are an essential tool in SQL that helps developers improve code readability and maintenance of complex queries.
By using CTEs instead of tables, developers can speed up the database’s performance, keep a cleaner database, and make it easier to understand how queries relate to the database schema.
We recommend learning CTEs to those who deal with long queries that are hard to understand, manage, and maintain.
Learning provides a deeper insight into how queries work and how CTEs can help you achieve effective coding through intuitive names of temporary result sets. Some good resources for learning CTEs include LearnSQL.com courses, which cover topics such as Recursive Queries, Creating Basic SQL Reports, and interactive lessons on SQL basics.
The key takeaway is that CTEs can be a powerful tool in your SQL arsenal, especially when dealing with complex queries, and should be considered in lieu of creating tables unnecessarily. In conclusion, the use of Common Table Expressions (CTEs) in SQL can significantly improve code readability, maintainability, and efficiency.
By using CTEs instead of subqueries or tables, developers can break down complex queries into smaller, more manageable parts, minimize unnecessary metadata, and make querying more efficient. Multiple CTEs are particularly useful in long and complicated queries and for hierarchical data structures.
CTEs are an essential tool in SQL that developers should use to achieve effective coding through intuitive names of temporary result sets. By learning how to use CTEs, developers can gain a deeper understanding of how queries relate to the database schema, resulting in more efficient and effective coding practices.