Combining Results in SQL: Mastering the Use of UNION in MySQL

The data university has arrived, and today we are enrolled in a special session on a powerful SQL tool in MySQL: the command UNION. This command is essential when we need to combine results from two or more different queries into a single result set, keeping the database organized and efficient.

What is UNION in MySQL?

In MySQL, UNION is an operator used to combine the results of two or more queries SELECT in a single set of results. It is essential when we work with data that is in separate tables, but shares a similar nature and we want to analyze them together.

How does UNION work?

The operator UNION takes the results of several queries and joins them together. So that UNION works correctly, it is essential that each query SELECT inside of the UNION have the same number of columns in the result, with compatible data types and in the same order. But what if we want to include duplicate results as well? Here comes another variant: UNION ALL.

UNION vs UNION ALL

Within join operations, MySQL provides us with two flavors:

• UNION: Remove duplicate rows between result sets.
• UNION ALL: Includes all rows from all result sets, including duplicates.

The use of one or the other will depend on the specific need of our consultation or analysis.

Practical Examples of Using UNION

To better understand how to implement UNION, let's see some practical examples. Imagine that you have two tables: Products y Services. Both tables have columns for ID, Name y Price, although they represent different types of offers from your business.

Combining Products and Services

Suppose we want to obtain a combined list of names and prices of both products and services. This is where UNION comes to the rescue.

SELECT Name, Price FROM Products UNION SELECT Name, Price FROM Services;

This simple example will return a list, without duplicates, with the names and prices of products and services.

Considering All Elements, Duplicates Included

Now, if we wanted to include all elements, duplicates included, this is how we would change our approach:

SELECT Name, Price FROM Products UNION ALL SELECT Name, Price FROM Services;

Advanced Cases Using UNION

UNION It is not limited to simple examples. It can be extremely useful in more complicated queries, such as combining data from multiple tables that have different sets of information but compatible structures.

Filtering and Combining in Complex Queries

Imagine you want to combine products and services, but only those that are priced above a certain threshold. Here we could use conditions within our queries before the join.

SELECT Name, Price FROM Products WHERE Price > 100 UNION SELECT Name, Price FROM Services WHERE Price > 100;

Best Practices and Considerations

When using UNION, there are some best practices and considerations we should keep in mind to keep our queries efficient and effective:

1. Proper indexing: Make sure the columns used in the clauses WHERE of your queries SELECT are properly indexed.
2. Data type consistency: Verify that data types are compatible between all the queries you are joining to avoid errors and unexpected behavior.
3. Order and limit: You can use ORDER BY to organize the results of the union and LIMIT to restrict the number of rows returned.

What's next after mastering UNION?

To dominate UNION y UNION ALL in MySQL can open doors to more complex and efficient data analysis where it is crucial to combine multiple sources of information in one place. Continue exploring more SQL functions and operators, and when you need extra help or want to share your experiences, visit my blog where we frequently discuss these and other related topics.

In conclusion, the command UNION MySQL is a powerful and flexible tool for manipulating and analyzing data from multiple queries. Its correct use not only allows us to simplify and enhance our queries, but also helps us keep our data organized and accessible. In case of doubts or to delve into other topics, do not hesitate to contact me. Happy coding!