UNION vs UNION ALL
UNION removes duplicate result rows; UNION ALL keeps every row.
UNION
Use it when
Prefer UNION ALL when duplicates are valid or already controlled.
UNION ALL
Use it when
Use UNION only when duplicate removal is part of the requirement.
Decision rule
Choose the option that matches the real requirement, data size, maintainability needs and compatibility constraints. Avoid selecting a technique only because it is familiar.
Need clarity?Prefer the simpler option that expresses the business rule directly.
Verify before scaling
Run both approaches on a small known dataset if the difference affects financial, inventory or operational decisions. Equivalent-looking techniques can behave differently with duplicates, blanks, NULL values or version constraints.