SQL Semantics

SELECT Keeps Duplicates

Plain SELECT projection keeps bag multiplicity.

highlighted = computed this step

SELECT projects a bag

A plain SELECT projection keeps duplicate projected rows. This differs from Book One projection, which returned a set. Note: duplicate result rows are visible only in the render.

select keeps bags\text{select keeps bags}

Projected duplicates remain

The recomputed row count is 5. Note: projection changed the columns but did not opt into deduplication.

rows=5\text{rows}=5

SQL bag/multiset + three-valued NULL logic, deterministic but surprising; tiny finite tables; no engine/perf claims. Set algebra is Book 1.

SELECT keeps duplicates select course arity 1 rows 5 Grades arity 3 rows 5
select course DB DB DB OS OS

Summary

Plain SELECT is bag projection, so multiplicity survives. Note: this bag behavior and NULL behavior are the subject here; set algebra was Book One.

select summary\text{select summary}