How Can I Efficiently Join Four SQL Tables Using IDs to Combine Data?

Efficiently join multiple SQL tables using IDs

Joining multiple SQL tables is a basic operation in data integration and analysis. When there are columns with matching values ​​in the table, you can join them using IDs and retrieve data from multiple data sources.

Suppose you have four tables: TableA, TableB, TableC, and TableD, with the following column structure:

• TableA: aID (primary key), nameA, dID
• TableB: bID (primary key), nameB, cID, aID
• TableC: cID (primary key), nameC, date
• TableD: dID (primary key), nameD

You want to join these tables to extract and combine data. You have successfully joined TableA, TableB and TableC using their respective IDs:

<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>

However, when you try to add TableD to the connection, you get an error because "TableD" is not recognized.

To resolve this issue you need to add another connection to contain TableD. The correct query should be:

<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 query performs a four-table join, starting with TableA and joining other tables in sequence based on matching IDs. This allows you to retrieve data from all four tables and combine them into a single result set.

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