SQL, or Standard Query Language, is a programming language for managing and manipulating data within a relational database management system (RDBMS). It’s a standard language used in many companies to help businesses access data smoothly. Because it’s widely used, the employment usually cites SQL as one of the necessary skills. That’s why it’s essential to learn about SQL. One of the common problems for people when learning SQL is understanding the queries, mainly when another person writes them. We would work as a team in the companies and often need to read and understand their SQL queries. So, we need to practice deconstructing the SQL queries and understand them. This article will walk through a step-by-step process to read and understand SQL queries.
The first thing we need to do when encountering an SQL query is to understand the general intention of the SQL queries. The general intention doesn’t mean we know entirely about the query’s structure; it’s more about the overall flows. For each clause, their functions are: SELECT (What columns are to be taken from the table), FROM (Which table the data came from), JOIN (Combine tables with the specified identifier), WHERE (Data filter based on the condition), GROUP BY (Organize the data based on the column’s value and allow aggregation function to be performed), ORDER BY (Arrange the data result order based on the specific column), and HAVING (The filter condition for aggregation function that cannot be specified with WHERE). When you look at the query above, try to identify the standard clauses. The clause would provide you with an understanding of what data are selected (SELECT), where it is from (FROM and JOIN), and the condition (WHERE, GROUP BY, ORDER BY, and HAVING).
There would be a time when you would encounter a complex query where so many SELECT clauses exist in the same query. In this case, we should understand the final result of the query or the final (first) SELECT you see in the query. The key is to know what the query output wants to be.
There would be a time when you would encounter a complex query where so many SELECT clauses exist in the same query. In this case, we should understand the final result of the query or the final (first) SELECT you see in the query. The key is to know what the query output wants to be. The query seems more complex and longer now, but the initial focus should be on the final SELECT, which seems to try to produce the customer’s total spending and purchase history. Try to assess what the final result wants to be and break it down from there.
After understanding how the final result should be and where it comes from, we need to look closer at the details. From here, we would backtrack into each subqueries and understand why they are structured like that. However, we don’t try to look at them in the top-down structures. Instead, we should try to look at the subqueries that are closer to the final result, and we move up to the one furthest from the final result.
Reading and understanding SQL queries is a skill that everyone should have in the modern era, as every company deals with them. By using the following step-by-step guide, you would have a better time to understand the complex SQL queries. The steps include:
1. Understand the General SQL Query Structure
2. Understand the Final Select
3. Understand the Final Conditions Clause
4. Understand the Final Join
5. Reverse Order Reading and Repeat.
Cornellius Yudha Wijaya is a data science assistant manager and data writer. While working full-time at Allianz Indonesia, he loves to share Python and Data tips via social media and writing media.