5 Data
Every entry filed under Data and its subtopics.
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- You are new to this topic.
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32 entries match these filters
- 0292 What is a database? A database is an organised store of data that programs can search, update and share safely, even with many users at once. Beginner · Explains an idea · Any system Article Choosing a database 15 Oct 2026
- 0298 What is JSON? JSON is a simple text format for structured data, made of names, values, lists and nested objects, used everywhere on the web. Beginner · Explains an idea · Any system Article CSV & JSON 15 Oct 2026
- 0299 What is YAML? YAML is a human friendly text format for configuration files where indentation shows structure instead of brackets. Beginner · Explains an idea · Any system Article CSV & JSON 15 Oct 2026
- 0293 What is SQL? SQL is the language you use to ask a relational database questions and to add, change or delete its data. Beginner · Explains an idea · Any system Article Queries 15 Oct 2026
- 0295 What is a transaction? A transaction groups several database changes so they all happen together or not at all, keeping the data consistent. Beginner · Explains an idea · Any system Article Queries 15 Oct 2026
- 0296 What is a primary key? A primary key is the column that uniquely identifies each row in a table, like an ID number nobody else can have. Beginner · Explains an idea · Any system Article Queries 15 Oct 2026
- 0297 What is a foreign key? A foreign key is a column that points to the primary key of another table, linking related rows together. Beginner · Explains an idea · Any system Article Joins 15 Oct 2026
- 0294 What is an index (database)? A database index is a lookup structure that lets the database find rows fast instead of reading the whole table. Beginner · Explains an idea · Any system Article Indexes & performance 15 Oct 2026
- 0260 Design and build a library database in SQLite: tables, keys, joins, reports and backups Model books, members and loans, create tables with keys and constraints, load sample data, write join and aggregate reports, add indexes checked… Beginner · Guided project · Any system Article Joins · Queries 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 · Cheat sheet · Any system Article 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 · Cheat sheet · Any system Article Basics 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 · Explains an idea · Any system Article Choosing a database · Database operations 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 · Cheat sheet · Any system Article Administration · Queries 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 · Fixes an error · Any system Article Roles & administration · Security & access 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 · Step by step · Any system Script Administration · Security & access 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 · Fixes an error · Any system Article Administration · Security & access 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 · Explains an idea · Any system Article Basics · Choosing a database 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 · Cheat sheet · Any system Article Install & connect · Roles & administration 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
- 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 · Explains an idea · 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 · Fixes an error · Any system Article Common errors · CSV & JSON 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 · Explains an idea · Any system Article Queries 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 · Step by step · Any system Script CSV & JSON · Files & automation 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 · Fixes an error · Any system Article Queries 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 · Cheat sheet · Any system Script Joins · Queries 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
- 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 · Step by step · Any system Article Data analysis · Files & automation 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
- 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 · Explains an idea · Any system Article Joins 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 · Explains an idea · Any system Article Indexes & performance 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 · Step by step · Any system Article Queries 15 Oct 2026
- 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 · Step by step · Any system Script CSV & JSON · Files & automation 15 Oct 2026