5 Data
Every entry filed under Data and its subtopics.
70 entries match these filters
- 0009 CSV to JSON converter A small command-line tool that turns a CSV file into a JSON array of objects, with optional number detection, custom delimiters and… Beginner · How-to · Any system Script CSV & JSON · Files & automation 15 Oct 2026
- 0064 jq: command-line JSON processor jq is a small command-line tool for reading, filtering and transforming JSON, like sed and awk for structured data. Linux, macOS, Windows Repo Command-line tools · jq & JSON on the command line 15 Oct 2026
- 0067 Your first SQL queries: SELECT, WHERE, ORDER BY, LIMIT and GROUP BY Read data from a table, keep only the rows you want, sort them, take the first few, and summarize groups with COUNT,… Beginner · How-to · Any system Article Queries 15 Oct 2026
- 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
- 0070 Indexes and EXPLAIN: making SQL queries fast An index is a sorted lookup structure that lets the database jump to matching rows instead of reading the whole table. EXPLAIN… Advanced · Concept · Any system Article Indexes & performance 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
- 0072 CSV vs JSON: choosing a format and avoiding the classic pitfalls CSV is a simple table of text; JSON holds nested, typed data. Both break in predictable ways: quoting, delimiters, encodings, numbers and… Beginner · Concept · Any system Article CSV & JSON 15 Oct 2026
- 0073 Fix: json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0) Python's json module found nothing it could parse at the very first character. The input is usually empty, HTML, or already consumed.… Beginner · Fix · Any system Article Common errors · CSV & JSON 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
- 0075 CSV to JSON converter (Python, standard library) Convert any CSV file with a header row into a JSON array or JSON Lines. Handles quoted fields, a byte order mark,… Beginner · How-to · Any system Script CSV & JSON · 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
- 0077 SQL cheat sheet: a runnable reference for everyday queries One SQL file that builds a tiny database and demonstrates filtering, sorting, grouping, joins, CASE, CTEs, window functions, safe updates and indexes.… Beginner · Reference · Any system Script Joins · Queries 15 Oct 2026
- 0078 pandas The most widely used Python library for working with tabular data: DataFrames, file readers and writers, grouping, joining and time series. Any system Repo Data analysis 15 Oct 2026
- 0079 DuckDB An in-process analytical SQL database. Run fast SQL directly on CSV, Parquet and JSON files, from the command line or inside Python… Any system Repo Data analysis · Queries 15 Oct 2026
- 0080 SQLite (official Git mirror) The source tree of SQLite, the self-contained SQL database engine embedded in phones, browsers and countless applications, mirrored to GitHub by its… Any system Repo Queries · SQL 15 Oct 2026
- 0198 Install PostgreSQL and connect with psql on Debian, Ubuntu and RHEL Install the PostgreSQL server from your distribution or the official PGDG repositories, start it, and open your first psql session as the… Beginner · How-to · Linux Article Install & connect 15 Oct 2026
- 0199 psql meta-commands cheat sheet The backslash commands you use every day in psql: connecting, listing databases, tables and roles, describing objects, timing queries, expanded output and… Beginner · Reference · Any system Article Install & connect · Roles & administration 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
- 0202 Back up and restore PostgreSQL with pg_dump, pg_restore and pg_dumpall Make logical backups of one database with pg_dump, of roles and the whole cluster with pg_dumpall, and restore them with pg_restore or… Beginner · How-to · Linux Article Backup & restore 15 Oct 2026
- 0203 Physical backups and point-in-time recovery with pg_basebackup and WAL archiving A base backup plus a continuous archive of WAL files lets you restore a PostgreSQL cluster to any moment after the backup,… Advanced · Concept · Linux Article Backup & restore · Replication & high availability 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
- 0205 Set up PostgreSQL streaming replication with a hot standby Step-by-step: prepare a primary, clone it with pg_basebackup, start a read-only standby that streams WAL, check it works, and understand promotion and… Advanced · How-to · Linux Article Replication & high availability 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
- 0208 Check PostgreSQL replication lag with a monitoring script A bash check that detects whether it runs on a primary or a standby, measures replication lag in bytes and seconds, and… Advanced · How-to · Linux Script Replication & high availability 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
- 0211 Fix: psql connection to server on socket /var/run/postgresql/.s.PGSQL.5432 failed psql tried the local Unix socket and found nothing there. The server is not running, listens on another port, or uses a… Beginner · Fix · Linux Article Install & connect 15 Oct 2026
- 0212 postgres/postgres: PostgreSQL source code mirror The official read-only GitHub mirror of the PostgreSQL source repository. Browse the code and history here; patches go through the project's mailing… Linux, macOS, Windows Repo Install & connect · PostgreSQL 15 Oct 2026
- 0213 pgBackRest: backup and restore for PostgreSQL pgBackRest is a widely used PostgreSQL backup tool with parallel full, differential and incremental backups, WAL archiving, encryption and S3, Azure or… Linux Repo Backup & restore 15 Oct 2026
- 0214 MySQL users and privileges: CREATE USER, GRANT and authentication plugins A MySQL account is a user name plus the host it connects from. You create it with CREATE USER, give it rights… Beginner · How-to · Linux Article Administration · Security & access 15 Oct 2026
- 0215 mysql command-line client cheat sheet The connection options, built-in commands and everyday SQL you need in the mysql (and mariadb) command-line client, on one page. Beginner · Reference · Any system Article Administration · Queries 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
- 0217 MySQL binary logs and point-in-time recovery The binary log records every change after your last backup. Restore the backup, then replay the binary log up to just before… Advanced · Concept · Linux Article Backup & restore · Replication 15 Oct 2026
- 0218 Setting up MySQL source-replica replication with GTIDs Step by step: configure a MySQL 8.4 source and replica with GTIDs, create the replication account, copy the data, connect the replica… Advanced · How-to · Linux Article Administration · Replication 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
- 0220 Fix: ERROR 1045 (28000): Access denied for user ‘appuser’@’localhost’ (using password: YES) The server rejected the user, host and password combination. Usually it is a wrong password, a host part that does not match,… Beginner · Fix · Linux Article Administration · Security & access 15 Oct 2026
- 0221 Oracle Database basics: instance vs database, CDB and PDB An Oracle instance is memory and processes; the database is the files on disk. Since 21c every database is a container database… Beginner · Concept · Linux, Windows Article Concepts & tools 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
- 0224 RMAN backup basics: ARCHIVELOG mode, a nightly backup script and restore Enable ARCHIVELOG mode, set an RMAN retention policy, run a nightly full backup with archived logs from cron, and learn the restore… Advanced · How-to · Linux Script Administration · Backup with RMAN 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
- 0228 MySQL Server source code Oracle's public GitHub repository with the source code of MySQL Server, including the server, client programs such as mysql and mysqldump, and… Linux, macOS, Windows Repo MySQL & MariaDB 15 Oct 2026
- 0229 MariaDB Server The source of MariaDB Server, the community-developed fork of MySQL started by original MySQL developers, licensed under GPLv2. Linux, macOS, Windows Repo MySQL & MariaDB 15 Oct 2026
- 0230 Oracle docker-images: Oracle Database containers Oracle's official collection of container build files and examples for its products, including Dockerfiles and a build script for Oracle Database single-instance… Linux Repo Concepts & tools · Oracle Database 15 Oct 2026
- 0231 MongoDB documents and collections compared with SQL tables MongoDB stores data as JSON-like documents grouped in collections instead of rows in tables. This explains the model, the _id field, embedding… Beginner · Concept · Any system Article Basics · Choosing a database 15 Oct 2026
- 0232 mongosh cheat sheet: CRUD, indexes and explain() The mongosh commands you use every day: switching databases, inserting, querying, updating and deleting documents, creating indexes and checking a query plan… Beginner · Reference · Any system Article Basics 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
- 0236 Redis data types and commands: a quick reference Redis stores values under keys, and each value has a type: string, hash, list, set, sorted set or stream. The essential commands… Beginner · Reference · Any system Article 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
- 0239 Fix: (error) NOAUTH Authentication required. (and securing Redis with ACLs) Redis requires a login and your client sent a command without one. Pass the right user and password, and use the moment… Advanced · Fix · Linux Article Administration & persistence · Security & access 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
- 0241 Choosing a database: relational, document or key-value Relational databases, document stores like MongoDB and key-value stores like Redis each fit different jobs. An honest look at what each does… Beginner · Concept · Any system Article Choosing a database · Database operations 15 Oct 2026