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• 0068 SQL joins explained: INNER, LEFT and finding rows with no match A join combines rows from two tables using a matching condition. INNER JOIN keeps only matches, LEFT JOIN keeps every row from… Intermediate · Concept · Any system Article Joins 15 Oct 2026
• 0069 NULL in SQL: why = NULL never matches and how to handle missing values NULL means "unknown", so comparisons with it are neither true nor false. Use IS NULL, COALESCE and careful aggregates to avoid rows… Intermediate · Concept · Any system Article Queries 15 Oct 2026
• 0071 Fix: column “x” must appear in the GROUP BY clause or be used in an aggregate function PostgreSQL rejects a grouped query that selects a column with more than one possible value per group. Add the column to GROUP… Intermediate · Fix · Any system Article Queries 15 Oct 2026
• 0074 pandas basics: read a CSV, filter rows and summarize with groupby Load a CSV into a pandas DataFrame, inspect it, filter rows with conditions, and compute totals per group, with the SQL equivalent… Intermediate · How-to · Any system Article Data analysis · Files & automation 15 Oct 2026
• 0076 jq recipes: 23 one-liners for JSON on the command line A runnable collection of jq filters for everyday JSON work: pick fields, filter with select, reshape, sort, count, group, pass shell variables,… Intermediate · Reference · Linux, macOS, Windows Script Command-line tools · jq & JSON on the command line 15 Oct 2026
• 0200 PostgreSQL roles, privileges and default privileges Create login and group roles, grant access to databases, schemas and tables, and use ALTER DEFAULT PRIVILEGES so tables created later get… Intermediate · How-to · Linux Article Roles & administration · Security & access 15 Oct 2026
• 0201 pg_hba.conf and PostgreSQL authentication methods pg_hba.conf decides who may connect from where and how they must prove who they are. Rules are checked top to bottom and… Intermediate · Concept · Linux Article Install & connect · Security & access 15 Oct 2026
• 0204 VACUUM, autovacuum and table bloat in PostgreSQL Updates and deletes leave dead row versions behind. VACUUM makes that space reusable, keeps statistics fresh and prevents transaction ID wraparound; autovacuum… Intermediate · Concept · Linux Article Performance & tuning · Roles & administration 15 Oct 2026
• 0206 Nightly PostgreSQL backup script with pg_dump and rotation A bash script for cron that dumps cluster roles plus every database in custom format into a dated folder, checks each archive,… Intermediate · How-to · Linux Script Backup & restore 15 Oct 2026
• 0207 Find long-running PostgreSQL queries and slow statements A psql-based script that lists sessions whose current query or open transaction is older than a threshold, plus pg_stat_statements queries to find… Intermediate · Reference · Linux Script Performance & tuning · Roles & administration 15 Oct 2026
• 0209 Fix: FATAL: password authentication failed for user PostgreSQL reached the server and found a matching pg_hba.conf rule, but the password check failed. Usually a wrong password, a role without… Intermediate · Fix · Any system Article Roles & administration · Security & access 15 Oct 2026
• 0210 Fix: FATAL: no pg_hba.conf entry for host The server received the connection but no pg_hba.conf rule matches this client address, user, database and SSL mode. Add or correct a… Intermediate · Fix · Linux Article Install & connect · Security & access 15 Oct 2026
• 0216 MySQL vs MariaDB: what actually differs MariaDB started as a fork of MySQL and still speaks the same client protocol, but GTIDs, JSON storage, authentication and tooling have… Intermediate · Concept · Linux Article Choosing a database · MySQL & MariaDB 15 Oct 2026
• 0219 mysqldump nightly backup with rotation (bash) A cron-ready bash script that dumps each MySQL or MariaDB database to its own compressed file with --single-transaction, verifies it and deletes… Intermediate · How-to · Linux Script Administration · Backup & restore 15 Oct 2026
• 0222 Connecting to Oracle: SQL*Plus, SQLcl, EZConnect, tnsnames.ora and the listener Clients reach an Oracle database through the listener, using an Easy Connect string or a tnsnames.ora alias. Here are the formats, the… Intermediate · Reference · Linux, macOS, Windows Article Administration · Concepts & tools 15 Oct 2026
• 0223 Oracle users, roles and tablespaces: setup script and usage report (SQL) A SQL*Plus/SQLcl script that creates a tablespace, a role and an application user inside a PDB, then reports tablespace usage from the… Intermediate · How-to · Any system Script Administration · Security & access 15 Oct 2026
• 0225 Oracle Data Pump: export and import schemas with expdp and impdp Data Pump moves schemas, tables or whole databases between Oracle databases as dump files. Set up a directory object, export with expdp… Intermediate · How-to · Linux, Windows Article Administration · Backup with RMAN 15 Oct 2026
• 0226 Fix: ORA-12541: TNS:no listener The client reached the host but nothing was listening on that port. Start or fix the Oracle listener, or correct the host… Intermediate · Fix · Linux, Windows Article Administration · Concepts & tools 15 Oct 2026
• 0227 Fix: ORA-01017: invalid username/password; logon denied Oracle rejected the login. Check the password (case-sensitive), that you connect to the right PDB service, and whether the account is locked… Intermediate · Fix · Any system Article Administration · Security & access 15 Oct 2026
• 0233 Turn on MongoDB authentication and create least-privilege users A fresh MongoDB install accepts connections without a password. Create a user administrator, enable authorization, then give each application and backup job… Intermediate · How-to · Linux Article Administration · Security & access 15 Oct 2026
• 0234 Nightly mongodump backup with rotation A bash script for cron that writes a compressed mongodump archive each night, keeps the password out of the process list, checks… Intermediate · How-to · Linux Script Administration · Database operations 15 Oct 2026
• 0235 Fix: MongoServerSelectionError: connect ECONNREFUSED 127.0.0.1:27017 Your app or tool tried to reach MongoDB on port 27017 and nothing accepted the connection. Usually mongod is not running, crashed… Intermediate · Fix · Linux, macOS, Windows Article Administration · Basics 15 Oct 2026
• 0237 Use Redis as a cache: TTLs, maxmemory and an eviction health report Set a memory limit and an eviction policy so Redis behaves like a cache, give keys TTLs, then run a bash report… Intermediate · How-to · Linux, macOS Script Administration & persistence · Basics 15 Oct 2026
• 0238 Redis persistence and high availability: RDB, AOF, replicas and Sentinel Redis keeps data in memory and writes it to disk with RDB snapshots, an append-only file, or both. Replicas copy data to… Intermediate · Concept · Linux Article Administration & persistence 15 Oct 2026
• 0240 Fix: MISCONF Redis is configured to save RDB snapshots, but it’s currently unable to persist to disk Redis blocks writes because its last background snapshot failed. Find the cause in the log (full disk, permissions, failed fork), fix it,… Intermediate · Fix · Linux Article Administration & persistence 15 Oct 2026
• 0243 Database backup strategy: the 3-2-1 rule and testing restores Keep three copies of your data on two kinds of storage with one copy offsite, choose dump or snapshot methods per database,… Intermediate · How-to · Linux Article Database operations · Security & access 15 Oct 2026
• 0250 Containerize a Node.js app and run it with PostgreSQL using Docker Compose Package a small Node.js web app in a multi-stage Docker image, then run it next to PostgreSQL with Docker Compose, named volumes,… Intermediate · Tutorial · Linux, macOS, Windows Article Docker basics · Docker Compose · PostgreSQL 15 Oct 2026
• 0257 Run PostgreSQL properly on a Linux server: install, lock down, back up, monitor and patch Set up PostgreSQL 18 from the official repository for one application: a dedicated role and database, SCRAM logins from one app server… Intermediate · Tutorial · Linux Article Backup & restore · Install & connect · Roles & administration 15 Oct 2026
• 0259 Speed up a slow database query with a Redis cache: cache-aside in Python, measured and locked down Put Redis in front of a slow query in a small Python app: cache-aside reads, TTLs with jitter, invalidation on writes, an… Intermediate · Tutorial · Linux Article Administration & persistence · Basics 15 Oct 2026