Author: web-admin

Choosing the right data type for each column is like choosing the right size of box when packing things for a move: Too small → your data gets cut off or you get errors...

Constraints are the rules you place on your columns to make sure the data inside your tables is correct, consistent, and meaningful. Think of them as the “guardrails” that prevent bad data from entering...

Up until now, we’ve been architects: designing databases, creating tables, choosing perfect data types, and adding strong constraints. Now it’s time to become data entry specialists (but the smart, efficient kind 😄). In SQL,...

Almost everything you do in SQL revolves around retrieving data — asking the database questions and getting exactly the answers you need. And the star of this chapter is the SELECT statement — the...

Now we’re going deep into the WHERE clause — the part that lets you filter exactly which rows you want to see. Everything we do here is about writing conditions that the database checks...

Up until now, we’ve been looking at individual rows — one book, one author, one order at a time. Now we’re going to summarize huge amounts of data: How many books do we have...

Up until now, we’ve been working with one table at a time. But in real life, data is almost always spread across multiple tables (customers in one table, orders in another, products in a...

A subquery (also called a nested query) is simply a SELECT statement inside another SELECT statement. It’s like asking a question, and inside the answer you ask another smaller question. Subqueries make SQL incredibly...

Set operations let you combine the results of two or more SELECT queries as if they were mathematical sets. Think of each SELECT as producing a “bag of rows”. With set operations, you can:...

Up until now, we’ve been creating tables, inserting data, and reading it with SELECT. But real databases are alive — prices change, stock gets updated, customers cancel orders, old records get archived… That’s what...