LINQ Join and GroupJoin: Inner Join, Composite Keys and the Left Join Pattern
How Join, as an inner join, drops unmatched rows and multiplies multiple matches, and its documented order. The direction of equals in query syntax and CS1937/CS1938, composite keys and CS1941, hierarchical results with GroupJoin, the left join pattern with GroupJoin + DefaultIfEmpty and its null trap, multiple joins, and how Join runs in n + m with a lookup.
7 Nov 2026
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11 min
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