A modern guide to SQL JOINs

SQL JOINs are best understood by starting with LEFT JOIN and focusing on the N:1 case (primary key on the right side). Always use ID equality in ON conditions and move other filters to WHERE clauses. The 1:N case of LEFT JOIN produces mixed results that combine data and metadata awkwardly. INNER JOIN is a filtered Cartesian product and is symmetrical, but maintaining discipline in syntax helps when switching between JOIN types. Self-joins work identically to regular joins but require table aliasing. Understanding these patterns prevents common performance issues and logic errors.