Data Engineering Spark SQL - Windowing Functions - Order of execution of SQL

In this article, we will explore the order of SQL execution, breaking down each step to help you understand how SQL queries are processed. We will accompany this explanation with practical examples and tasks for hands-on practice.

Key Concepts Explanation

Order of Writing the Query

When writing an SQL query, it is essential to follow a specific order to ensure correct results:

  1. Select
  2. From
  3. Join or Outer Join with On
  4. Where
  5. Group By and optionally Having
  6. Order By

Let’s delve into each of these steps with detailed explanations and code examples to better understand the order of writing a query.

SELECT

The SELECT statement retrieves data from a database and allows you to specify which columns you want to retrieve. Here’s an example:

SELECT column1, column2
FROM table_name;

FROM

The FROM clause indicates the tables from which the data will be retrieved. It is used to specify the data source in your query. For example:

SELECT column1, column2
FROM table_name;

JOIN

The JOIN clause is used to combine rows from two or more tables based on a related column between them. It can be either an inner join or an outer join. Here’s an example:

SELECT column1, column2
FROM table1
JOIN table2 ON table1.column_name = table2.column_name;

WHERE

The WHERE clause is used to filter records based on specific conditions. It acts as a filter to retrieve only the records that meet the criteria specified. Example:

SELECT column1, column2
FROM table_name
WHERE condition;

GROUP BY

The GROUP BY clause groups rows that have the same values into summary rows. It is often used with aggregate functions like SUM, COUNT, AVG, etc. Example:

SELECT column1, COUNT(column2)
FROM table_name
GROUP BY column1;

ORDER BY

The ORDER BY clause is used to sort the result set in ascending or descending order based on one or more columns. Example:

SELECT column1, column2
FROM table_name
ORDER BY column1 ASC;

Hands-On Tasks

Now that we have covered the key concepts of SQL query execution, let’s practice writing and executing SQL queries following the order explained above. Here are some tasks for you to complete:

  1. Write a query that retrieves specific columns from a table.
  2. Write a query that performs a join operation between two tables.

Conclusion

In conclusion, understanding the order of SQL query execution is crucial for writing efficient and effective queries. By following the prescribed steps, you can ensure that your queries return accurate results. Remember to practice these concepts and engage with the community for further learning.

Order of execution of SQL

Let us review the order of execution of SQL queries and understand the importance of following a specific sequence when writing queries. Remember, the order matters!

Watch the video tutorial here