How to Join Tables Lacking Direct Foreign Key Relationships Using an Intermediate Table?

Connecting Tables Without Direct Foreign Keys: Leveraging an Intermediate Table

Joining tables lacking direct foreign key relationships can be tricky. The solution? An intermediary table containing foreign keys from both tables.

Consider three tables: Species, Forest, and Tree. The Tree table links to both Species and Forest via foreign keys, but Species and Forest don't directly reference each other.

Query: Joining Species and Forest Data

To retrieve forest names, species names, and wood types for forests in the 'ARCTIC' region, use this SQL query:

SELECT Forest.Fo_name, Species.Sp_name, Species.Sp_woodtype
FROM Forest
INNER JOIN Tree ON Tree.Tr_forest = Forest.Fo_name
INNER JOIN Species ON Tree.Tr_species = Species.Sp_name
WHERE Forest.Fo_loc = 'ARCTIC'
ORDER BY Forest.Fo_name, Species.Sp_name;

Breakdown of the Query

This query efficiently joins the tables:

1. It first joins Forest and Tree using Tree.Tr_forest (the forest name in the Tree table) and Forest.Fo_name (the forest name in the Forest table).
2. Then, it joins Tree and Species using Tree.Tr_species (the species name in the Tree table) and Species.Sp_name (the species name in the Species table).
3. The WHERE clause filters the results to show only forests located in 'ARCTIC'.
4. Finally, ORDER BY sorts the results by forest name and then species name.

This method effectively uses the Tree table as a bridge to connect Species and Forest, enabling the retrieval of the desired information.

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