Welcome to ORDER BY jungle
The article from BoringSQL delves into the intricacies of SQL's ORDER BY clause, a seemingly simple feature that can lead to significant issues if not properly understood. Key points include the handling of NULL values, where different databases (e.g., PostgreSQL, MySQL, SQL Server) have varying defaults—NULLs may sort first or last. Collation and locale settings also affect string sorting, potentially causing non-intuitive orderings. Performance considerations are discussed, such as the impact of sorting on large datasets and the use of indexes to optimize ORDER BY operations. The article provides concrete examples, like how MySQL's default NULL ordering (NULLs first) differs from PostgreSQL's (NULLs last), and advises developers to explicitly specify NULLS FIRST or NULLS LAST for portability. It also covers the use of ORDER BY with expressions, functions, and CASE statements, emphasizing the need for careful testing across database systems.
Understanding ORDER BY nuances prevents subtle bugs in data presentation and query performance.