SQL Constraints Explained (Primary Key, Foreign Key, Unique, Check)
SQL constraints explained: PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, CHECK and DEFAULT, what each enforces, with CREATE TABLE examples and referential integrity.
Explore the latest SQL tips and tutorials in our blog section. Learn SQL programming, database management, and query optimization.
SQL constraints explained: PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, CHECK and DEFAULT, what each enforces, with CREATE TABLE examples and referential integrity.
SQL NULL values explained: what NULL means, three-valued logic, testing with IS NULL, how NULL affects joins and aggregates, and COALESCE, IFNULL and NULLIF.
SQL operators explained for beginners: arithmetic, comparison, logical, BETWEEN, IN, LIKE and IS NULL, each with examples, results, and operator precedence rules.
SQL data types explained: numeric, string, date and time, boolean and JSON types, with examples, cross-database differences, and best practices for choosing them.
The five categories of SQL commands explained: DDL, DML, DCL, TCL and DQL, what each does, the commands it includes, and a clear example of every one.
What is SQL used for? Real-world examples across finance, healthcare, e-commerce and more, plus the roles that rely on SQL and how it fits the modern data stack.
SQL vs MySQL vs SQL Server vs PostgreSQL: what is a language versus a database system, the key syntax and feature differences, and how to choose the right database.
A library of practical, copy-paste SQL examples organized by task: filtering, joins, aggregation, subqueries, window functions, and reporting patterns, each with results.
SQL best practices for clean, fast, secure SQL: naming, formatting, constraints, data types, NULL handling, parameterized queries, transactions and indexing, with examples.
A complete SQL window functions guide: OVER, PARTITION BY, frames, ROW_NUMBER, RANK, LAG and LEAD, and running totals, with runnable examples and their results.