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Learn how to effectively join tables in DB2 to retrieve comprehensive data, including names and IDs, with an easy-to-follow query example.
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This video is based on the question https://stackoverflow.com/q/64798585/ asked by the user 'Syed' ( https://stackoverflow.com/u/3118692/ ) and on the answer https://stackoverflow.com/a/64798713/ provided by the user 'Fahmi' ( https://stackoverflow.com/u/6914864/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Join Tables in DB2 to Retrieve Data Accurately

Joining tables in a database can be a bit of a challenge, especially when you're trying to pull related data from different tables. If you're working with DB2 and you need to retrieve comprehensive information across multiple tables, you're in the right place.

In this guide, we will explore a common scenario where you need to join tables to get supervisor details along with personal information. Let's take a closer look at the problem and provide a clear solution.

The Problem: Retrieving Supervisor Information

You have two tables:

Table1: Personal Information

[[See Video to Reveal this Text or Code Snippet]]

Table2: Supervisor Information

[[See Video to Reveal this Text or Code Snippet]]

In this setup:

Table1 contains personal details of employees.

Table2 links employees with their supervisors.

The task is to retrieve a list that includes the employee's ID, name, supervisor's ID, and supervisor's name.

Sample Data Table

The desired output for employee 101 (Syed) should look like this:

[[See Video to Reveal this Text or Code Snippet]]

The Initial Query

You started with the following SQL query:

[[See Video to Reveal this Text or Code Snippet]]

While this query successfully retrieves the supervisor ID, it fails to get the supervisor’s name.

The Solution: A Corrected SQL Query

To retrieve both the supervisor ID and name correctly, you can use a slightly different approach. Here’s the SQL query you need:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Solution

Table Aliasing:

We use aliases (like t, t1, and t2) to make the query clearer.

t1 refers to the original employee information from Table1.

t2 refers to the supervisor information from Table1.

Joins:

First Join: We join Table2 with Table1 to match the l_id (the employee ID) with the employee information.

Second Join: We join again with Table1 (using a different alias t2) to find the supervisor name corresponding to the sup_id.

Selection of Columns:

We select the employee's ID, name, supervisor ID, and supervisor name to get the complete dataset.

Outcome

Executing this query will provide you with a result set that has the employee's personal information along with their supervisor's ID and name, thus ensuring you have all the necessary information in one output.

Conclusion

Joining tables in DB2 can seem daunting, but with a clear understanding of how to correlate the relationships between tables, you can easily retrieve the data you need.

The key steps are ensuring you're using the correct joins and aliases, which can greatly simplify your queries. With the SQL examples provided, you should now be able to pull the necessary information efficiently.

If you're still stuck or have further questions, feel free to ask for more assistance!