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Real data is rarely complete. A customer may not have provided a phone number, an order may not yet have a shipping date, and a book may not yet have been assigned a category. SQL uses a special marker, NULL, to represent this missing or unknown information. Working with NULL correctly is one of the […]
Removing data from a database sounds simple, but a single missing clause can wipe out an entire table. In this tutorial, you will learn how to use the SQL DELETE statement safely: how to target the exact rows you want to remove, how to verify your target before deleting, and how to use transactions to […]
Data stored in a database rarely stays the same forever. Prices change, customers move, orders are shipped, and inventory goes up and down. When you need to modify rows that already exist in a table, SQL provides the UPDATE statement. In this tutorial you will learn how to change the value of one or more […]
When you query a table, the result set often contains repeated values. For example, if you list the countries of every customer, the same country name appears many times. The DISTINCT keyword tells the database to return only unique values, removing duplicates from the result. In this tutorial you will learn what DISTINCT does, how […]
A SELECT statement can easily return thousands or millions of rows, but in most real applications you only need a handful of them at a time — the ten cheapest books, the top five customers, the most recent orders, or a single page of search results. Downloading everything and discarding the rest wastes memory, network […]
When you run a SELECT statement, the database does not promise to return rows in any particular order. If you want a predictable sequence — the cheapest books first, the newest orders on top, or authors listed alphabetically — you have to ask for it explicitly. The tool SQL gives you for that is the […]
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Writer and founder of Diary Coding — sharing notes, tutorials, and experiences about programming for anyone who wants to keep learning.