Relating Multiple Keys in One Table to Keys in Another Table Using a Single Query

In this article, we will explore how to perform a single query to relate multiple keys from one table (access) to keys in another table (usersx). We will use MySQL as the database management system for this example.

Understanding Joins

Before diving into the solution, let’s briefly discuss joins. A join is used to combine rows from two or more tables based on a related column between them. There are several types of joins, including:

• Inner Join: Returns only the rows that have a match in both tables.
• Left Join (or Left Outer Join): Returns all the rows from the left table and matching rows from the right table. If there’s no match, the result is NULL on the right side.
• Right Join (or Right Outer Join): Similar to left join but returns all rows from the right table.

In our example, we will use an Inner Join to combine rows from access and usersx based on matching keys (owner_id and date_created_id).

Relating Keys Using a Single Query

The problem you’re facing is that MySQL doesn’t support joining multiple tables with the same key using a single query. However, we can use two joins to achieve this result.

Problem

Let’s analyze the given code snippet:

SELECT usersx.last_name 
FROM usersx
JOIN access 
ON access.owner_id = usersx.id AND access.date_created_id = usersx.id
WHERE access.owner_id = '123' OR access.date_created_id = '246';

As mentioned earlier, you can’t have a user row that matches both owner_id and date_created_id. To fix this issue, we need to join twice: once for owner_id, and once for date_created_id.

Solution

We’ll use two inner joins to combine rows from access with matching keys in usersx. The query will look like this:

SELECT o.last_name AS owner_last_name,
       d.last_name AS date_created_last_name
FROM access a
INNER JOIN usersx o ON o.id = a.owner_id
INNER JOIN usersx d ON d.id = a.date_created_id
WHERE a.owner_id = 123 OR a.date_created_id = 246;

In this query:

• We join usersx with access twice using inner joins.
• The first join (o) is used for matching owner_id.
• The second join (d) is used for matching date_created_id.

By joining twice, we can get both last names from the usersx table that match owner_id and date_created_id in the access table.

Explanation

Let’s consider an example to understand how this works:

Suppose you have owner_id = 123 and date_created_id = 246 in the access table. The query will join with the following rows:

• From usersx, we get a row with id = 456.
  • Join 1 (o) uses id = 456 to match owner_id.
  • Result: A row from usersx with last_name = ‘Smith’.
• From usersx, we get a row with id = 567.
  • Join 2 (d) uses id = 567 to match date_created_id.
  • Result: A row from usersx with last_name = ‘Jones’.

By combining these rows, the query will return both last names (‘Smith’ and ‘Jones’).

Best Practices

When using multiple joins, make sure to:

• Specify the correct join type (inner, left, right) based on your requirements.
• Use meaningful table aliases for clarity.
• Include relevant conditions in the WHERE clause to filter rows.
• Optimize queries by avoiding unnecessary joins or using indexes.

Conclusion

Relating multiple keys from one table to keys in another table can be challenging. By understanding how to use inner joins and combining them with proper logic, you can achieve this result using a single query. This article demonstrated how to join two tables based on matching keys and return desired data.

Last modified on 2023-11-07