- MySQL – Scheduled Events
Cron inside the database. The event_scheduler variable that is OFF by default so your first event never runs, CREATE EVENT with AT and with EVERY, STARTS and ENDS, ON COMPLETION PRESERVE, altering and disabling one, where errors go, what happens on a replica, and the question to settle first — whether this belongs in the database at all or in the scheduler you already operate.
- LeetCode 118 – Pascal's Triangle
The gentlest bottom-up DP there is, and on the list as the setup for its follow-up. The edges are where the rule runs out of inputs rather than a special case bolted on, the output size IS the complexity so there is nothing to optimise — which is exactly why problem 119 asks for one row.
- MySQL – Triggers
Running a statement automatically when a row changes. BEFORE and AFTER, INSERT, UPDATE and DELETE, the NEW and OLD rows, writing an audit trail, keeping a derived total in step, the limitations that bite — a trigger cannot touch its own table, and it does not fire for TRUNCATE — and the real argument against them: logic that runs invisibly is logic nobody debugging your application will think to look for.
- MongoDB – Query Operators
Comparison, logical, element and array operators, plus projection — the vocabulary every later query is built from.
- LeetCode 114 – Flatten Binary Tree to Linked List
The best problem on the list for the idea that pointer surgery can replace a data structure. Three solutions using O(n), O(h) and O(1) space — the last one splices the right subtree onto the left subtree's rightmost node, which is Morris threading kept rather than undone.