Backend Development › Relational Databases & SQL
SQL
The language for querying relational databases.
Also known as: Structured Query Language, SQL language, SQL queries, sequel
SQL (Structured Query Language) is the standard language for working with relational databases. You use it to read data, change it, and define the tables themselves. It has been around for decades, and it’s one of the most useful skills for anyone who works with data.
SELECT name, COUNT(*) AS orders
FROM customers c
JOIN orders o ON o.customer_id = c.id
WHERE o.status = 'paid'
GROUP BY name
ORDER BY orders DESC
LIMIT 10;SQL is declarative: you describe what result you want, not how to compute it. The database’s query planner decides how, such as which indexes to use and in what order to join (query plan).
The families of statements
| Group | Does | Examples |
|---|---|---|
| DQL (queries) | Read | SELECT |
| DML (manipulation) | Change rows | INSERT, UPDATE, DELETE |
| DDL (definition) | Change structure | CREATE TABLE, ALTER TABLE, DROP |
| DCL / TCL | Permissions and transactions | GRANT, COMMIT, ROLLBACK |
(See SQL sub-languages.)
One standard, many dialects
SQL is standardized, but each database (PostgreSQL, MySQL, SQL Server, Oracle, SQLite, BigQuery, Snowflake…) adds its
own functions, syntax and quirks: LIMIT vs TOP, date functions, string concatenation, UPSERT syntax. The core
(SELECT, JOIN, GROUP BY, WHERE) carries across. Look up the specifics for yours (common databases).
Why it matters
- It’s how most applications persist their data, so backend developers use it daily, often through an ORM that writes it for them. Knowing it makes the ORM easier to reason about.
- Data engineers and analysts use it as their main tool.
- Frontend developers benefit when they understand what the API’s data model is.
Where to start
Learn in this order: SELECT / WHERE / ORDER BY, JOIN, GROUP BY, NULL behavior, subqueries and CTEs, then window functions and indexes. Practice on a real database, since reading about queries only goes so far.