Adventures in Machine Learning

Mastering the Art of SQL: Applying UNION and UNION ALL

Applying UNION and UNION ALL using SQL

When working with SQL, it’s essential to understand how to apply and use the UNION and UNION ALL operators, as they allow you to combine data from two or more tables into a single table. Both operators have their uses, but it’s important to know the difference between them to get the most out of your SQL queries.

In this article, we will explore how to apply UNION and UNION ALL to our SQL queries.

Steps to apply UNION using SQL

UNION is a SQL operator that combines the result sets of two or more SELECT statements. The result set of UNION contains only distinct rows that appear in either result set of the SELECT statements.

Here are the steps to apply UNION using SQL:

Locating the tables

To begin, we need to locate the tables we want to combine. The tables should have the same columns or compatible data types to be combined into one result set.

Applying UNION using SQL

Next, we need to apply UNION using SQL. We start by using the SELECT statement to query the columns we want to include in the result set.

For example, suppose we have two tables called “sales_2019” and “sales_2020”, and we want to combine the sales data into a single table. We can use the following SQL query:

SELECT product_name, sale_date, sale_amount

FROM sales_2019

UNION

SELECT product_name, sale_date, sale_amount

FROM sales_2020;

This SQL query will give us a result set that contains the product names, sale dates, and sale amounts from both tables.

Applying conditions under the Union

We can also apply additional conditions under the UNION operator to filter or sort the result set. For instance, we can use the WHERE clause to only select rows that meet certain criteria, and the ORDER BY clause to sort the result set in ascending or descending order based on a column.

Steps to apply UNION ALL using SQL

UNION ALL is a SQL operator that combines the result sets of two or more SELECT statements, but unlike UNION, it includes all rows, including duplicates, that appear in either result set of the SELECT statements. Here are the steps to apply UNION ALL using SQL:

Locating the tables

As with UNION, we need to locate the tables we want to combine. The tables should have the same columns or compatible data types to be combined into one result set.

Applying UNION ALL using SQL

Next, we need to apply UNION ALL using SQL. We start by using the SELECT statement to query the columns we want to include in the result set.

For example, suppose we want to combine two tables called “customers_us” and “customers_europe” into a single table. We can use the following SQL query:

SELECT customer_id, first_name, last_name, country

FROM customers_us

UNION ALL

SELECT customer_id, first_name, last_name, country

FROM customers_europe;

This SQL query will give us a result set that contains the customer IDs, first names, last names, and countries from both tables, including duplicates.

Applying conditions under the Union

We can also apply additional conditions under the UNION ALL operator to filter or sort the result set, as we did with UNION.

Conclusion

In conclusion, understanding how to apply and use the UNION and UNION ALL operators in SQL is critical for combining data from multiple tables into a single result set. By following the steps outlined in this article, you can combine tables and apply conditions to the result set to get the most out of your SQL queries.

With practice, you can become proficient in using these operators and optimize your SQL queries for maximum efficiency and functionality. 3)

Applying UNION ALL using SQL

In SQL, the UNION ALL operator is used to combine the result sets of two or more SELECT statements. The difference between UNION and UNION ALL is that UNION only includes unique or distinct rows from the result set, while UNION ALL includes all rows, even duplicates.

This article will explore how to apply UNION ALL in SQL and compare it to UNION.

Applying UNION ALL using SQL

To apply UNION ALL using SQL, first, we need to locate the tables we want to combine. The tables should have the same columns or compatible data types to be combined into one result set.

Once we have that, we can use the following SQL query:

SELECT column1, column2, column3, …

FROM table1

UNION ALL

SELECT column1, column2, column3, … FROM table2;

This SQL query will give us a result set that contains all rows from both tables, including duplicates.

The important thing to note is that the columns in both SELECT statements must be in the same order, and they should contain the same data type.

Comparing UNION ALL to UNION

To better understand the difference between UNION ALL and UNION, let’s consider an example. Suppose we have two tables called “employees_2019” and “employees_2020”, and we want to combine the two tables to get a list of all the employees from both years.

Using UNION, we can use the following SQL query:

SELECT emp_id, first_name, last_name

FROM employees_2019

UNION

SELECT emp_id, first_name, last_name

FROM employees_2020;

This SQL query will give us a result set that contains all unique employees from both tables. If there are duplicate employee records between the two tables, UNION will only return one of them.

On the other hand, using UNION ALL, we can make the same SQL query:

SELECT emp_id, first_name, last_name

FROM employees_2019

UNION ALL

SELECT emp_id, first_name, last_name

FROM employees_2020;

This SQL query will give us a result set that contains all employees from both tables, including duplicates. If there are duplicate employee records between the two tables, UNION ALL will return both of them.

The difference between UNION ALL and UNION is that UNION returns only the distinct rows from the result set, while UNION ALL returns all rows, including duplicates.

Conclusion and additional resources

In conclusion, UNION ALL is a useful operator in SQL for combining the result sets of two or more SELECT statements into a single table. Compared to UNION, UNION ALL includes all rows, even duplicates, in the result set.

By understanding how to apply UNION ALL and how it differs from UNION, you can better optimize your SQL queries to meet your needs. If you’re looking to learn more about SQL, there are many online resources available.

Some popular SQL tutorials include W3Schools, SQLZoo, and Codecademy. With practice and a willingness to learn, you can become proficient in SQL and use it to extract insights from your data.

In conclusion, understanding how to use and apply the UNION and UNION ALL operators in SQL is crucial for working with multiple tables and combining data sets. UNION allows you to combine only distinct rows, while UNION ALL includes all rows, even duplicates.

By following the steps outlined in this article, you can confidently use these operators to query your data and optimize your SQL queries for maximum efficiency and functionality. With the abundance of online resources available, such as SQL tutorials, there is no excuse not to further develop SQL skills.

Being proficient in SQL can lead to valuable insights from your data and help make informed business decisions.

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