How to Efficiently Join Four SQL Tables Using Shared IDs?

Connecting Multiple SQL Tables via Shared IDs

Working with relational databases often requires combining data from multiple tables. This involves joining tables based on common columns. This guide demonstrates how to effectively join four tables (TableA, TableB, TableC, and TableD) using their respective IDs.

Table Structures:

• TableA: aID | nameA | dID
• TableB: bID | nameB | cID | aID
• TableC: cID | nameC | date
• TableD: dID | nameD

Initial Join (Tables A, B, and C):

A common method to join Tables A and C through Table B uses the shared "aID" and "cID" columns:

<code class="language-sql">SELECT TableA.*, TableB.*, TableC.*
FROM (TableB INNER JOIN TableA ON TableB.aID = TableA.aID)
INNER JOIN TableC ON TableB.cID = TableC.cID
WHERE DATE(TableC.date) = DATE('now()')</code>

Adding TableD to the Join:

Directly joining TableD to the initial query is problematic due to a lack of a shared column. The solution is to add another join condition:

<code class="language-sql">SELECT TableA.*, TableB.*, TableC.*, TableD.*
FROM TableA
JOIN TableB ON TableB.aID = TableA.aID
JOIN TableC ON TableC.cID = TableB.cID
JOIN TableD ON TableD.dID = TableA.dID
WHERE DATE(TableC.date) = DATE('now()')</code>

This improved query connects all four tables using their shared ID columns. Note the improved readability achieved through proper formatting and indentation.

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