- 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.
- Oracle Database – Data Types
VARCHAR2 versus CHAR and why the BYTE/CHAR length qualifier matters in UTF-8. NUMBER precision and why money never goes in a binary float. The fact that DATE includes a time, and the range comparison that follows from it. Booleans before 23ai, the empty string that is NULL, and a full mapping table to Java types.
- Rate Limiting
Four algorithms and what each one does at the boundary: fixed window and its doubling bug, sliding log, sliding window counter, and the token bucket that wins most of the time. Where the limiter belongs, how to identify a caller without handing them a way to reset their own quota, why the counter must be atomic, and the 429 contract a client can actually back off against.