Database Management Systems · topic
DDL and DML, the logical evaluation order of a SELECT, SQL joins, duplicates and DISTINCT, set operations with bag counts, NULL and three-valued logic, aggregates and GROUP BY / HAVING, and nested or correlated subqueries with IN, EXISTS, ANY and ALL, with results run on SQLite and PostgreSQL.
SQL is the standard language of relational databases. It has a data definition part (DDL) that creates and changes schemas and constraints, a data manipulation part (DML) that inserts, updates and deletes rows, and a query part built around the SELECT ... FROM ... WHERE block.
Most real queries combine tables with joins, summarise rows with aggregate functions, and must cope with missing values. These three features interact: an outer join manufactures NULLs, aggregate functions skip NULLs, and the WHERE and HAVING filters drop anything whose condition is UNKNOWN.
A subquery is a SELECT inside another statement. It lets a query compare a value with a computed set ("salary greater than every salary in EE"), test whether related rows exist ("departments with no employee"), or express "for all" conditions without a division operator.
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