5.1 SQL

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• 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
• 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
• 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 · Tutorial · Any system Article Joins · Queries 15 Oct 2026