SQL for developers - from SELECT to advanced queries

In a world where data is referred to as the „new oil”, the ability to manage it effectively is absolutely fundamental to the work of a software engineer. Whether you’re building a simple mobile app or a complex analytics system, you need to understand, how SQL databases work. SQL (Structured Query Language) is a universal language that has been the standard for communicating with relational databases such as PostgreSQL, MySQL and SQL Server for decades.

SQL Basics: Retrieving and Filtering Data

Working with SQL starts with understanding that data in relational databases is stored in structured tables. Your task as a programmer is to extract exactly the information your application needs from them, as quickly as possible.

Every programmer’s essential toolkit:

• SELECT: Specify which columns you want to retrieve.
• WHERE: Filtering records based on specific criteria (e.g. active users only).
• ORDER BY: Sorting results (e.g. by most recent orders).
• GROUP BY and aggregate functions: Calculating totals, averages or counting records (e.g. total sales for each region).

If you’re wondering whether a relational database or a document-based solution would be better for your project, take a look at our comparison: MongoDB vs PostgreSQL – when should you choose which one?.

Joining tables: The power of JOINs

In relational databases, information is spread across multiple tables to avoid duplication (a process known as normalisation). To get the full picture – for example, a list of orders along with the customers’ names – you need to use a JOIN operation.

1. INNER JOIN: It returns only those records that have matches in both tables.
2. LEFT JOIN: Returns all records from the left table and matching records from the right table. If there is no match, it inserts NULL.
3. RIGHT and FULL OUTER JOIN: Less commonly used, but essential in specific analytical reports for identifying missing links.

Understanding, how SQL databases work In the context of relationships, it allows you to build efficient structures that don’t become bloated as the number of users grows. This is crucial when you’re planning Building a REST API with a database.

Advanced SQL: Window Functions and CTEs

When simple queries are no longer enough, modern SQL comes into play. Window functions and common table expressions (CTEs) are the elements that distinguish a junior developer from an experienced one.

• CTE (WITH clause): It allows you to create clear, temporary result sets that you can use in your main query. This is a much better alternative to nested subqueries, which quickly become difficult to read.
• Window functions (e.g. ROW_NUMBER(), RANK(), SUM() OVER()): They allow you to perform calculations on a set of rows associated with the current row, without having to group them into a single row (as GROUP BY does). They make it easy to create rankings or running totals.

You can find practical exercises using these functions at SQLZoo – interactive SQL exercises, where you can test queries „live”.

Optimisation: Why does my query take 10 seconds?

Writing correct queries is only half the battle. The real challenge is writing fast queries. When your table orders will grow to several million records, a standard SELECT without proper optimisation, it may cause the application to crash.

How can you optimise SQL databases?

1. Indexing: Indexes are like a table of contents in a book. They allow a database to find a record instantly without having to search through the entire table. Remember, though: too many indexes slow down writes (INSERT/UPDATE).
2. Analysis of the query plan: Use the command EXPLAIN ANALYZE (in PostgreSQL) or EXPLAIN (in MySQL). This will allow you to see how the database „thinks” and where it spends the most time (e.g. during a so-called sequential scan).
3. *Avoiding SELECT: Only retrieve the columns you actually need. This reduces the load on the network and RAM.

You can find detailed technical guidelines on performance in the documentation PostgreSQL – official documentation.

SQL is a language that will stay with you for years to come

Front-end technologies change every few years, but relational databases and SQL have been the cornerstone of the industry since the 1970s, and there is no indication that this is set to change. Understanding this, how SQL databases work, ...will not only help you write better code, but also debug data issues more effectively and deliver faster applications.

At 4ADStudio, we know that data is at the heart of every application. We design and optimise databases to ensure they stand the test of time and can handle millions of queries.

Is your database running too slowly? Do you need to optimise your queries or design a new data architecture? Get in touch with us – our SQL specialists will ensure your data works to help you succeed!