- AWS – Stop an EC2 Instance on a Schedule with Lambda
A dev instance running nights and weekends is roughly 70% waste. Twenty lines of boto3, an EventBridge schedule, and an IAM policy scoped to a tag instead of `*`. Why the handler filters on instance state before it calls stop, what the stopping → stopped transition means for your EBS bill, and the mistake that stops your production fleet: a tag filter that matches nothing is a filter that matches all.
- Oracle Database – Joins
ANSI joins, the legacy (+) operator you will meet in every pre-2005 codebase and why missing one silently turns an outer join into an inner one, semi-joins with EXISTS, and the NOT IN trap that returns zero rows without an error when the subquery contains a NULL. Plus the correlated join a plain JOIN cannot do, the three join methods Oracle picks between, and how to recognise a bad nested loop in a plan.
- Oracle Database – SELECT Essentials
The querying idioms that are specific to Oracle: DUAL, FETCH FIRST versus the ROWNUM pagination trap, NVL against COALESCE, NULLS FIRST, date format masks where MM and MI are not the same thing, LISTAGG and its 4000-byte overflow, ROLLUP, MERGE as the upsert, and INSERT ALL because Oracle rejects a multi-row VALUES list.
- Oracle Database – Tables, Constraints and Sequences
A CREATE TABLE worth copying, and the five constraint types. Why an unindexed foreign key locks the child table, why composite UNIQUE lets partly-null duplicates through, and the ON UPDATE CASCADE Oracle does not have. Identity columns versus sequences, CACHE and the gaps it creates, and how to drop a column on a large table.
- AWS – ECR: The Registry Your Containers Come From
Every container you run on ECS, EKS or Lambda is pulled from a registry, and on AWS that is ECR. Authenticating with a token that expires in twelve hours, the tag immutability setting that stops `:latest` meaning two different things, lifecycle policies so old images do not bill forever, and the vulnerability scan worth turning on. Plus the pull that fails from a private subnet, and why it is a VPC endpoint.