A decomposition of a relational schema whose component relational projections always reconstruct the original database relation by natural join, without extra database tuples.
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Lossless join decomposition is a concept in database normalization that ensures that when a relation (table) is decomposed (i.e., broken into two or more smaller relations), you can reconstruct the original relation without losing any information. In simpler terms, if you take a database table and split it into smaller tables, a lossless join decomposition means that you can join those smaller tables back together to get the exact original table back.