How to Correctly Join Multiple Tables in SQL with Identical Foreign Keys?

Joining Multiple Tables with SQL Inner Join

When working with multiple tables in a database, it's often necessary to join them to retrieve related data. One type of join is called an inner join, which retrieves only rows that match the specified criteria.

To join multiple tables, you can use the following syntax:

SELECT *
FROM table1
INNER JOIN table2 ON table1.column1 = table2.column2
INNER JOIN table3 ON table1.column1 = table3.column3

In this example, the table1 table is joined to the table2 table using the column1 column, and then to the table3 table using the column1 column again. This ensures that only rows that match the specified criteria in all three tables are returned.

However, you may encounter an issue when trying to join multiple tables with the same foreign keys. For instance, suppose you have the following schema:

table1: id, name
table2: id, table1Id
table3: id, table1Id

You can join the first two tables with the following query:

SELECT * FROM `table1` INNER JOIN `table2` ON table1.id=table2.table1Id

Now, you want to join the third table into the query. You might try the following:

SELECT * FROM `table1` 
INNER JOIN `table2` 
INNER JOIN table3 
ON table1.id=table2.table1Id=table3.table1Id

However, this query will not return any results. The reason is that the table1.id column is being compared to both table2.table1Id and table3.table1Id. To fix this, you need to ensure that each table is joined using its own unique foreign key column. The correct query would be:

SELECT * 
FROM table1 
INNER JOIN table2
      ON table1.id=table2.table1Id
INNER JOIN table3
      ON table1.id=table3.table1Id

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