Advanced SQL Techniques for Data Engineering: Window Functions, Common Table Expressions, and More


Window Functions, Common Table Expressions (CTEs), and other complex methods are used in Advanced SQL approaches for Data Engineering to manipulate data in SQL databases. With the help of window functions, data may be split up and arranged in a variety of ways, enabling sophisticated computations and data aggregations. Common Table Expressions (CTEs) offer a method for streamlining complex queries by dividing them into more manageable, smaller chunks. This can improve query performance while also making it simpler to write and manage queries. Subqueries, Pivot Tables, and Recursive Queries are more sophisticated approaches. Data engineering problems including data transformation, data cleaning, and data aggregation can be solved using these strategies. By leveraging advanced SQL techniques, data engineers can gain more insights from their data to make better decisions and improve their business.

Scope of the article

  • In this article we will first with the introduction to advanced SQL techniques for data engineering.
  • In this article we will also read about the Explanation of Window Functions, including syntax, functions, and examples of use cases for performing complex queries, ranking, and aggregations.
  • We will also read about the common Table Expressions (CTEs), including syntax, examples, and best practices for creating recursive and non-recursive CTEs to simplify complex queries and improve performance.
  • We will read about the tips and tricks for optimizing SQL queries, including using indexing, partitioning, and parallel processing to speed up large queries and reduce latency.
  • At last we will read a small conclusion on the article topic and the takeaways that we have learned from the article.


Relational databases may be managed and queried with the help of the robust Structured Query Language (SQL) programming language. It has become more crucial for data engineers to be knowledgeable in sophisticated SQL procedures as the number and complexity of data continue to increase. Advanced SQL approaches here allude to things like Window Functions and Common Table Expressions (CTEs).

Data engineers can execute computations across windows, or sets of rows, in a table by using the potent tool known as window functions. This method enables engineers to do operations like value aggregation and running totals as well as more complicated searches.

For data engineers, common table expressions (CTEs) are a crucial tool. CTEs are transient result sets that can be used as references in a more comprehensive SQL statement. This method enables easier code reading and more effective inquiries.

Subqueries, intricate joins, and employing indexes for performance optimisation are other advanced SQL techniques. To ensure that queries run effectively and rapidly, these strategies call for a thorough understanding of SQL syntax.

For data engineers who want to work with huge and complicated datasets, advanced SQL approaches are crucial. Engineers may efficiently analyse and manipulate data using Window Functions, Common Table Expressions, and other methods, which enables them to obtain deeper insights into the data kept in databases.

Explanation of Window Functions

A powerful SQL approach for carrying out sophisticated queries, ranking, and aggregations on a set of data is called window functions. A window function operates on a “window,” or subset of data, within a query result set and makes calculations depending on that window.
The syntax for a window function is as follows:

FUNCTION_NAME (argument) OVER ([PARTITION BY partition_expression, ... ]
                             [ORDER BY sort_expression [ASC | DESC], ... ]
                             [ROWS or RANGE frame_specification])

The window function, such as SUM, AVG, MAX, MIN, or RANK, is identified by its FUNCTION_NAME. The column or expression that the function operates on is the argument. The ORDER BY clause sorts the rows within each partition in a certain order, whereas the PARTITION BY clause separates the result set into groups or partitions based on the supplied expression(s). The subset of rows within the window, referred to as a “frame,” over which the function executes is specified by the ROWS or RANGE clause.

Ranking is one of the most frequent applications for window functions, where you want to give each row in a result set a rank or row number based on a particular column or expression. For instance, the RANK() function gives each row a distinct rank depending on the values of a given column.

Aggregations, in which you want to perform computations based on a subset of rows in a result set, are another application for window functions. The COUNT() function, which counts the number of rows in the window, as well as the SUM() and AVG() functions are frequently used for this purpose.

The Function window can also be used to calculate running totals or running averages, which are sums or averages of a particular row or expression for the current row. For example, the SUM() ROW (START FROM ROW ASC ROWS BETWEEN UNBOUNDED PREPEDING AND CRRENT ROW ) function sums the continuation counts of the specified rows, while the AVG() CLOSED (START FROM ROW ASC BETWEEN UNBOUNDED PREPEDING AND CURRENT) function calculates the running average.

Common Table Expressions (CTEs)

CTEs are an effective SQL tool for simplifying complex queries and enhancing performance. In the context of a single SQL statement, a CTE is a temporary named result set that is defined. For data engineers that need to execute complicated queries and data transformations, it can be used to reference and alter the result set in subsequent SQL statements.

The syntax for creating a CTE is as follows:

WITH cte_name (column_name1, column_name2, ...) AS (
    SELECT column_name1, column_name2, ...
    FROM table_name
    WHERE conditions
SELECT column_name1, column_name2, ...
FROM cte_name;

A list of column names is followed by a definition of the CTE and a name assignment in the WITH clause. The SELECT statement that the CTE uses to retrieve data from one or more tables and filter the resulting set is introduced by the AS keyword.

In order to process hierarchical or recursive data structures, recursive queries—which refer back to themselves—can be made using CTEs, which is one of its main advantages. A base case, which serves as the recursion’s starting point, and a recursive case, which represents the recursion’s recurring element, are the two components that make up recursive CTEs.

WITH top_employees AS (
    SELECT employee_name, salary
    FROM employees
    ORDER BY salary DESC
    LIMIT 10
SELECT employee_name, salary
FROM top_employees;

And here is an example of a recursive CTE that selects all the descendants of a given employee in a hierarchical data structure:

WITH RECURSIVE employee_hierarchy AS (
    SELECT employee_name, manager_name
    FROM employees
    WHERE employee_name = 'John Smith'
    SELECT e.employee_name, e.manager_name
    FROM employees e
    JOIN employee_hierarchy eh ON e.manager_name = eh.employee_name
SELECT employee_name
FROM employee_hierarchy;

It’s crucial to adhere to best practises while developing CTEs in order to maximise performance and preserve readability. This includes limiting the CTE’s size to save memory, giving the CTE and its columns sensible names, and preventing circular references in recursive CTEs. Additionally, using indexes on the tables being queried is advised to enhance query performance.

Tips and tricks for optimizing SQL queries

Optimizing SQL queries is a critical aspect of data engineering, as it can have a significant impact on query performance and response times. There are several tips and tricks that data engineers can use to optimize their SQL queries, including indexing, partitioning, and parallel processing.

By building indexes on the columns that are frequently requested or utilised in joins, indexing is a usual approach used to enhance query performance. By enabling the database to rapidly find the pertinent rows in a table rather than having to scan the entire table, indexing can speed up query execution. As indexes can slow down data loading and insert operations, it is crucial to take into account the trade-off between query execution speed and index maintenance overhead when establishing indexes.

Another method for optimising SQL queries is partitioning, which splits big tables into smaller, more manageable segments. By reducing the quantity of data that needs to be scanned and enabling queries to run concurrently across numerous partitions, partitioning can enhance query performance. Hash partitioning based on a specific column value or a range of values (such as dates or numbers) can both be used for partitioning.

Parallel processing is a technique that can be used to speed up large SQL queries by dividing the workload across multiple CPU cores or nodes in a cluster. Parallel processing is particularly effective for queries that involve large data sets or complex joins, as it allows multiple tasks to be performed simultaneously, reducing query latency and improving throughput. Parallel processing can be achieved through a variety of techniques, such as using parallel query execution, using distributed databases or data warehouses, or using specialized hardware such as GPUs.

Other tips and tricks for optimizing SQL queries include:

  • Reducing the quantity of data that needs to be scanned and simplifying complex searches using subqueries or derived tables.
  • By avoiding pointless joins and, whenever possible, switching to inner joins from outside joins.
  • a table’s columns should have the appropriate data types and data structures to increase query performance and decrease storage needs.
  • Using the LIMIT or TOP keywords, or by specifying particular columns to obtain, one can restrict the quantity of data returned by a query.
  • Use of functions or expressions in the WHERE clause should be avoided since they can impede query performance.
  • Avoid retrieving all columns from a table using the SELECT * command as this can increase the quantity of data that needs to be scanned and reduce query performance.

Along with these methods and ideas, it’s crucial to keep an eye on query performance and examine execution plans to spot potential areas for improvement. The amount of data being scanned, the order of the joins, and the use of indexes and partitions are just a few of the details revealed by query execution plans on a query’s performance.


In conclusion, data engineers who wish to work with vast and complicated datasets must be familiar with advanced SQL techniques like Window Functions, Common Table Expressions, subqueries, and complex joins. These skills can be mastered through Scaler’s Data Scientist course, which will help engineers manipulate and analyze data more effectively and powerfully. This comprehensive training enables them to gain a deeper understanding of the dataset, extract valuable insights, and make informed decisions. Scaler’s Data Science course equips data engineers with the necessary tools to excel in their roles and unlock the full potential of complex data analysis.

Data engineers can generate running totals or spot trends in data using Window Functions, for instance, which let them to execute complex calculations over groups of rows. These capabilities may not be possible with simpler queries. Similar to how Common Table Expressions improve the efficiency and readability of queries by enabling engineers to build temporary result sets that may be used within a bigger SQL statement.

Understanding how to optimise queries for performance, such as employing indexes to speed up data retrieval or lowering the number of joins in a query, is also necessary in order to use these advanced SQL techniques.

Overall, for data engineers who want to flourish in today’s data-driven environment, learning sophisticated SQL procedures is a crucial skill. Engineers can have a better understanding of their data, make better decisions, and ultimately improve business outcomes by using these strategies.