SELECT, JOIN and GROUP BY retrieve and aggregate data.
SELECT retrieves data with WHERE, GROUP BY, HAVING and ORDER BY; JOINs (INNER, LEFT, RIGHT, FULL) combine tables; aggregates (COUNT, SUM, AVG); subqueries and window functions enable analytics. Avoid SQL injection by using parameterised queries.
SELECT c.name, SUM(o.total) FROM customers c JOIN orders o ON o.cid=c.id GROUP BY c.name.
SQL lets you ask questions of relational data, and joins and grouping turn raw tables into useful answers.
SELECT, JOIN and GROUP BY retrieve and aggregate data.
SQL lets you ask questions of relational data, and joins and grouping turn raw tables into useful answers.
It combines rows from two tables where the join condition matches, which lets you use related data in one result.
WHERE filters individual rows before grouping, while HAVING filters the groups after aggregation.
Typical applications include finding the total sales per region with GROUP BY, joining orders to customers to list who bought what and selecting the last hundred sensor readings from a table.
Study "Relational databases" first (topic 74 of 104 in Software & Computing). It covers the groundwork this topic builds on.
Query planner. Set operations. CTEs.
SELECT c.name, SUM(o.total) FROM customers c JOIN orders o ON o.cid=c.id GROUP BY c.name.
The next topic is "Normalisation", which builds directly on this one.
It is classed as Advanced in Software & Computing (topic 75 of 104). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Software & Computing.
Want to practise? Open the interactive course for quick checks, quizzes, simulations and progress tracking.