MySQL Tutorials

MySQL SQL

MySQL is a relational database management system. SQL lets an application create tables, insert records, retrieve data, update existing rows, and remove records. A reliable query names the table and columns clearly, filters deliberately, and uses parameters for user-supplied values.

SELECT name, price
FROM products
WHERE price > 500
ORDER BY price DESC
LIMIT 10;

This query selects specific columns, filters products, sorts the result, and limits the output. The individual lessons explain each clause and how clauses work together.

Changing data safely

INSERT INTO adds rows, UPDATE changes rows, and DELETE removes rows. Always list columns for inserts, preview the rows matched by a condition, and use a precise WHERE clause for updates and deletes. Transactions make related changes succeed or fail together.

Aggregation and joins

Aggregate functions such as COUNT(), SUM(), and AVG() summarize data. GROUP BY creates groups and HAVING filters those groups. Joins combine related tables through keys; choose INNER, LEFT, RIGHT, CROSS, or self joins according to how unmatched rows should be handled.

Database structure

Tables should use appropriate data types and constraints. Primary keys identify rows, foreign keys protect relationships, and NOT NULL, UNIQUE, CHECK, and DEFAULT rules protect valid values. Indexes can speed common lookups, but they also add storage and write cost. Views can provide a reusable read-only query interface.

Security and production practice

Never concatenate untrusted input into SQL. Use prepared statements, validate inputs, give application accounts only the privileges they need, protect credentials, and avoid exposing detailed database errors. Test schema changes and destructive statements on a safe copy before production.