SQL
PRIMARY KEY and FOREIGN KEY
The two constraints that give a database its structure: one that identifies a row, and one that guarantees a reference points at something real.
#Beginner
Suitable for beginners.
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SQL
NOT NULL, DEFAULT, UNIQUE and CHECK
The four constraints that guard a single table. What each one enforces, how NULL interacts with UNIQUE and CHECK, and why the database is the right pl...
HTML
Page Landmarks: header, nav, main and footer
Four elements divide a page into regions a screen reader can jump between. Learn the rules for each, including the one that says main appears exactly...
HTML
Labels: The Most Important Element in a Form
A label names a field for every reader and grows its click target. Two ways to bind one, and the reasons a form without labels is not usable.
HTML
Input Types: Text, Email, Number, Date and the Rest
One element, more than twenty types. Choosing the right one gives you the correct mobile keyboard, free validation and better autofill, at no cost.
HTML
Form Structure: form, action and method
Every form is a container, a set of named controls and a submit button. Learn what action and method do, why name matters more than id, and what the b...
SQL
LEFT JOIN and RIGHT JOIN
Outer joins keep unmatched rows and fill the missing side with NULL. Learn LEFT and RIGHT JOIN, and the one mistake that silently turns an outer join...
SQL
INNER JOIN Explained
INNER JOIN keeps only rows that match on both sides. Learn the syntax, how the join condition works, and why unmatched rows silently disappear.
HTML
Table Structure: table, tr, th and td
A table is for two dimensional data, never for layout. Learn the four core elements, the difference between th and td, and the test that tells you whe...
SQL
GROUP BY Fundamentals
GROUP BY collapses rows into one row per distinct key. Learn the single rule that governs the select list and how NULLs are grouped.
SQL
COUNT, SUM, AVG, MIN and MAX
The five aggregate functions that collapse many rows into one value, the three forms of COUNT, and what each one does with NULL.
HTML
Ordered and Unordered Lists
ul when the order does not matter, ol when it does. Plus the start, reversed, type and value attributes that most people never discover.