How to Select Distinct Employee Jobs from Multiple Tables in MySQL Using UNION

Описание: Learn how to effectively select distinct employee jobs from multiple tables in MySQL using the `UNION` operator to streamline your database queries and eliminate duplicates.
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This video is based on the question https://stackoverflow.com/q/62494327/ asked by the user 'owf' ( https://stackoverflow.com/u/12355557/ ) and on the answer https://stackoverflow.com/a/62494352/ provided by the user 'Tim Biegeleisen' ( https://stackoverflow.com/u/1863229/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Selecting Distinct Employee Jobs from Multiple Tables in MySQL

When working with databases, it’s common to encounter scenarios where you need to retrieve data from multiple tables based on certain criteria. In this post, we’ll explore a specific situation involving employee data from two different tables: regular_employee and contract_employee. We’ll learn how to effectively select distinct job titles for employees who joined in a specific month—in this case, February 2020.

The Challenge: Selecting Data from Multiple Tables

Imagine you have two tables in your MySQL database:

1. Regular Employees

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2. Contract Employees

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Your Goal

Retrieve a list of distinct employee_job titles for employees who joined in February 2020.

The Initial Approach: A Common Mistake

You might start with a query that attempts to pull the desired data using a JOIN, grouping by job titles from both tables. This could look something like this:

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However, this query will not yield the expected results. Instead of displaying unique job titles, it produces a combination of all jobs that meet the criteria, leading to duplicated values like this:

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This output is not useful if your goal is to list unique job titles.

The Solution: Using UNION to Retrieve Distinct Jobs

To achieve your goal efficiently, the UNION operator is the ideal solution. The beauty of UNION is that it automatically removes duplicate entries from the combined results of two SELECT queries.

Here’s the Correct Query:

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Explanation:

First SELECT: This retrieves employee_job titles from the regular_employee table for those who joined in February 2020.

UNION: This combines the results with the second SELECT query while ensuring duplicates are removed.

Second SELECT: This pulls employee_job titles from the contract_employee table, also filtering for February 2020.

Expected Result:

This query will produce the following clean output, listing each unique job title just once:

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

Conclusion

In this post, we discussed an effective method for selecting distinct employee job titles from multiple tables in MySQL. By utilizing the UNION operator, you can streamline your queries, eliminate duplicates, and efficiently retrieve the data you need. This technique is not only useful for employee data but can be applied in various scenarios across different database applications. Happy querying!