4 lessons
ACID and transactions: the four promises every database makes
What a transaction actually is, and the precise meaning of Atomicity, Consistency, Isolation and Durability.
Concurrency and isolation levels
Dirty reads, non-repeatable reads and phantom reads explained one at a time, and the four isolation levels that prevent (or allow) each.
Query execution and EXPLAIN: seeing what the database actually does
How a SQL query becomes a physical execution plan, reading EXPLAIN output, and the join algorithms the planner picks between.
Performance tuning, hands-on: fixing a genuinely slow query
A real slow query, diagnosed and fixed step by step using EXPLAIN, indexing, and query rewriting - tying the whole module together.