SQL
What Indexes Are and Why They Exist
An index is a sorted lookup structure the database maintains for you. Learn how a B-tree finds a row, what an index costs, and the clustered index in...
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SQL
Composite, Unique and Covering Indexes
Multi column indexes and why their order decides everything: the leftmost prefix rule, selectivity, covering indexes and index only scans.
SQL
When Indexes Help and When They Hurt
Indexes are not free. Learn the cases where the optimiser correctly ignores them, how to find unused and duplicate indexes, and how to maintain them.
SQL
How a Query Executes and Reading EXPLAIN
What happens between typing a query and getting rows, and how to read an execution plan - the single most useful performance skill in SQL.
SQL
Scans, Seeks, Join Strategies and Sorting
What the engine actually does: full scans versus index seeks, nested loop and hash joins, and why a sort or a temporary table appears in your plan.
SQL
Query Optimisation and Pagination Performance
A practical checklist for making a slow query fast, and the specific fix for deep pagination that no amount of indexing solves.
SQL
SQL Injection and Parameterised Queries
Why string concatenation lets user input rewrite your query, and why prepared statements make it structurally impossible - with the cases parameters c...
SQL
Users, Roles, GRANT, REVOKE and Least Privilege
Controlling who can do what: creating users, granting the minimum needed, grouping privileges into roles, and the accounts a typical application actua...
SQL
PostgreSQL, MySQL, SQL Server, Oracle and SQLite Compared
One reference table for the differences that actually break code: paging, auto increment, strings, dates, upserts and the features each product lacks.
SQL
Writing Portable SQL
How much portability is worth paying for, which constructs are safe everywhere, and how to isolate the parts that can never be portable.
SQL
Beginner Practice Queries
Twenty graded exercises on SELECT, filtering, sorting and simple functions, each with a worked solution and the reason it is written that way.
SQL
Intermediate Practice: Joins and Aggregation
Fifteen exercises combining joins, grouping and having, each with the solution and a note on the trap it was designed to expose.