Structured Query Language, the standard language for interacting with relational databases. It covers querying data with SELECT, modifying it with INSERT, UPDATE, and DELETE, defining schemas with CREATE and ALTER, and controlling access with GRANT and REVOKE. SQL is the universal interface to databases like PostgreSQL, MySQL, SQLite, and SQL Server.
NULL is the most misunderstood value in SQL. It is not zero. It is not an empty string. It is not a space. It is the absence of a value — the marker that says “this value is unknown, missing, or inapplicable.” Because NULL is not a value, it does not behave like one. Comparing…
A SELECT statement returns rows. Without an ORDER BY clause, the order of those rows is undefined. The database may return them in the order they were inserted, in the order they were found on disk, or in an order determined by the query plan. The order is not guaranteed, and it can change between…
A query can return millions of rows. The application rarely needs all of them. The user sees a page of ten or twenty results at a time. A dashboard shows the top five performers. A report shows the most recent hundred events. The LIMIT, OFFSET, and TOP clauses are the tools that control how many…
A query returns rows. Sometimes those rows contain duplicates. The DISTINCT clause removes them. It tells the database to return only the unique combinations of the selected columns. If two rows have the same values in every selected column, only one is returned. The DISTINCT clause is the tool for answering questions like “Which cities…
A table is empty when it is created. The INSERT INTO statement is the first DML statement that fills it with data. It adds one or more rows to a table. Every row that an application stores, every record that a user saves, every event that a system logs, arrives through an INSERT statement. It…