<h1>Union 3 Tables in PostgreSQL: Sorted by Date</h1>
<p>PostgreSQL is a powerful and versatile relational database management system that is widely used in many applications. One of its key features is the ability to combine data from multiple tables using the UNION operator. In this article, we will explore how to union three tables in PostgreSQL and sort the results by date.</p>
<h2>The Scenario</h2>
<p>Let's say we have three tables in our database: <strong>invoices, payments, </strong>and<strong> expenses.</strong> These tables contain information about our business transactions, such as invoice details, payment records, and expenses incurred. Each table has a <code>date</code> column that stores the date when the transaction occurred.</p>
<p>Our goal is to create a query that will combine the data from all three tables and sort it by date in ascending order. This will give us a complete and organized view of our business transactions.</p>
<h2>The UNION Operator</h2>
<p>The UNION operator is used to combine the results of two or more SELECT statements into a single result set. It is important to note that the SELECT statements must have the same number of columns and data types for the UNION operation to work.</p>
<p>In our scenario, we want to combine the data from the <strong>invoices, payments, </strong>and<strong> expenses </strong>tables, which have the same structure. Hence, we can use the UNION operator to achieve our goal.</p>
<h2>Combining the Data</h2>
<p>To combine the data from all three tables, we will use the following query:</p>
<pre><code>SELECT date, invoice_number, amount
FROM invoices
UNION
SELECT date, payment_number, amount
FROM payments
UNION
SELECT date, expense_number, amount
FROM expenses
ORDER BY date ASC;
</code></pre>
<p>In the first line of the query, we specify the columns that we want to retrieve from the <strong>invoices</strong> table. Then, we use the UNION operator to combine the results with the data from the <strong>payments</strong> table. Finally, we add another UNION statement to combine the data from the <strong>expenses</strong> table.</p>
<p>Notice that we have also added an ORDER BY clause to sort the results by date in ascending order. This is an important step to ensure that our data is organized properly.</p>
<h2>Executing the Query</h2>
<p>Now that we have our query ready, let's execute it and see the results. The output will be a single result set that contains all the data from the three tables, sorted by date.</p>
<pre><code>date | invoice_number | amount
-----+----------------+-------
Jan 1, 2021 | INV-001 | $100
Jan 5, 2021 | PAY-001 | $50
Jan 10, 2021 | EXP-001 | $20
Jan 15, 2021 | INV-002 | $150
Jan 20, 2021 | PAY-002 | $75
Jan 25, 2021 | EXP-002 | $30
Jan 30, 2021 | INV-003 | $200
</code></pre>
<p>As you can see, the results are sorted by date in ascending order, and we have a complete view of our business transactions from all three tables.</p>
<h2>Conclusion</h2>
<p>In this article, we have learned how to union three tables in PostgreSQL and sort the results by date. The UNION operator is a powerful tool that allows us to combine data from multiple tables, making it easier to analyze and organize our data. By using this technique, we can gain valuable insights and make informed business decisions.</p>
