Aprende UNIR 2 Tablas | Unión de Tablas

We've got the attention of a company that owns a small online store. They have 2 tables that are related to each other. The first table contains information about the products sold in the online store.

Here's what the product table looks like:

The second table contains product categories on the website, along with a brief description for each category.

Here's what the category table looks like:

Our first task is to join these two tables to find out how many products are in each category. We'll use a JOIN statement to achieve this.

Before diving into the task, let's understand what a JOIN statement is and how it works.

To join two tables, they need to share a common column. Let's see how JOIN works using the employees and department tables. Their common column is employees.department and departments.name.

Note

When writing columns from these tables, start with the table name, add a dot, and then the column name. This helps keep the code clear, especially when tables have columns with the same name. It tells SQL exactly which table and column you mean.

Para unir 2 tablas, deben tener una columna en común entre ellas. Consideremos el uso de JOIN usando el ejemplo de las tablas employees y department. Les recordaré que su columna común es employees.department y departments.name.

Nota

Fíjate cómo escribí las columnas de estas tablas. Primero, escribo el nombre de la tabla, luego pongo un punto y el nombre de la columna. Cuando usamos más de una tabla en una consulta, necesitamos especificar la tabla de la cual estamos tomando la columna para hacer el código legible. Además, en el caso de que las tablas tengan columnas con el mismo nombre, necesitamos que SQL entienda a qué tabla y columna nos estamos refiriendo.


              1234
            
SELECT department.type, SUM(employees.salary) AS total_salary
FROM employees
JOIN department ON employees.department = department.name
GROUP BY department.type

Let's break down how we used JOIN in our query:

In the SELECT part, we list the columns we want from both tables, making sure to include the table name for clarity;
In the JOIN part, we specify the table to join and the common column that links them. Here, it's employees.department and department.name;
We then group the data by type to calculate the total salary using the SUM() function.

If this seems complex, here's a simple syntax for using JOIN:

Vamos a entender paso a paso lo que hicimos y cómo utilizamos JOIN:

En la sección SELECT, especificamos las columnas que necesitamos recuperar de dos tablas, indicando el nombre de la tabla al que pertenece la columna;
En la sección JOIN, especificamos la tabla que queremos unir, seguido de especificar la columna común. En nuestro caso, es employees.department y department.name;
Luego, agregamos los datos por type (ya que tenemos una función de agregación SUM() en la sección SELECT para encontrar el salario total) y obtenemos el resultado que nos interesa.

La consulta puede parecer compleja, así que veamos la sintaxis general para usar JOIN:

Note

Unlike the UNION clause, the JOIN statement lets us combine entire tables, not just the columns they share. Plus, with JOIN, there's no need for subqueries like with UNION.

Tarea

Swipe to start coding

Your need to join the two tables: category and product. The common columns for these two tables are product.category_id and category.id.

Your task is to find the total amount of products in each category. To do this, you need to calculate the sum of the product.amount column.

Use the alias total_amount for this column. At the end of your query, sort the result by the total_amount column in ascending order.

In the response, you should have 2 columns: category.name and total.amount.

Brief Instructions

Retrieve the category.name column and the sum of the product.amount column from the product table.
Assign the alias total_amount to the second column.
Join the category table using a JOIN statement.
Match the tables on the common column product.category_id = category_id.
Group the results by category.name.
Sort the results by total_amount.

Solución

¿Todo estuvo claro?

¡Gracias por tus comentarios!

Sección 3. Capítulo 1

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Suggested prompts:

What are the different types of JOINs in SQL?

Can you explain how to join the product and category tables using SQL?

Why do we need to use table names before column names in JOIN queries?

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