Adventures in Machine Learning

UNION vs UNION ALL: Which SQL Command Should You Use?

Understanding

UNION vs.

UNION ALL: A Comprehensive Guide

Have you ever worked with large datasets and needed to combine two tables to get meaningful insights? Chances are you have come across the SQL

UNION operator.

UNION and

UNION ALL are two options available when combining two or more tables in SQL. They are commonly used in data manipulation tasks and are fundamental to the SQL language.

In this article, we will discuss the differences between

UNION and

UNION ALL, their syntax, usage rules, and when to use them efficiently. Let us dive in.

Union vs. Union All: What are they?

UNION and

UNION ALL are two SQL commands that combine two or more tables and return a single result set. The

UNION operator combines two tables, leaving out duplicate rows, whereas

UNION ALL combines two tables, including duplicate rows.

Differences between

UNION and

UNION ALL

The primary difference between

UNION and

UNION ALL is the inclusion of duplicate rows.

UNION excludes duplicate rows, whereas

UNION ALL includes duplicate rows. Nonetheless, both statements follow the same syntax and return similar results.

Syntax of

UNION and

UNION ALL

The syntax of

UNION and

UNION ALL is as follows:

SELECT column1, column2, … FROM table1

UNION/

UNION ALL

SELECT column1, column2, … FROM table2

The

UNION/

UNION ALL operator combines selected columns from two tables. The number of columns selected must be the same for both tables.

The use of aliases in the SELECT statement may be required when combining tables with the same column names.

Usage Rules for

UNION and

UNION ALL

When using

UNION and

UNION ALL, there are rules to be observed:

– The columns selected must have the same data type. – The order of columns in both tables must be the same.

– The number of columns in both tables must be the same. – The columns’ aliases should be unique.

Example of Using

UNION and

UNION ALL

Consider two clubs, “Book Club” and “Rowing Club,” with member’s names and their performance metrics. Members of Book Club:

– John

– Lisa

– Tim

Members of Rowing Club:

– Tim

– Sarah

– Lisa

Their performance metrics (in percentage) are as follows:

Book Club:

– John (90%)

– Lisa (70%)

– Tim (85%)

Rowing Club:

– Tim (80%)

– Sarah (60%)

– Lisa (70%)

To find out the top performers in both clubs, we use the

UNION and

UNION ALL operators. Using

UNION operator:

SELECT name, performance FROM book_club

UNION

SELECT name, performance FROM rowing_club;

Output:

– John (90%)

– Lisa (70%)

– Tim (85%)

– Sarah (60%)

Using

UNION ALL operator:

SELECT name, performance FROM book_club

UNION ALL

SELECT name, performance FROM rowing_club;

Output:

– John (90%)

– Lisa (70%)

– Tim (85%)

– Tim (80%)

– Sarah (60%)

As illustrated above,

UNION excludes duplicates, whereas

UNION ALL includes duplicates. Using

UNION and

UNION ALL Efficiently

When it comes to using

UNION and

UNION ALL efficiently, there are a few things to consider. Impact of

UNION on Performance

The use of

UNION can negatively impact performance.

The DISTINCT keyword is implied by

UNION and sorts and removes duplicates, which is an expensive operation. When to Use

UNION or

UNION ALL

In smaller datasets, the difference between

UNION and

UNION ALL is negligible. However, in larger datasets,

UNION ALL outperforms

UNION since

UNION requires an additional sort and removes the duplicates. Writing Complex Queries with

UNION and

UNION ALL

SQL Cookbook recommends using

UNION for recursive queries.

UNION is useful in handling hierarchical data, while

UNION ALL is best for handling large datasets that do not require duplicate removal.

Conclusion

In this article, we discussed the differences between

UNION and

UNION ALL, their syntax, usage rules, and when to use them efficiently. When combining two or more datasets,

UNION and

UNION ALL offers a straightforward and efficient way of getting meaningful insights from large datasets. Understanding the difference between the two commands is essential in making the right choice to achieve the desired results in your SQL tasks.

In summary,

UNION and

UNION ALL are two fundamental SQL commands used to combine two or more tables. The primary difference between the two is that

UNION excludes duplicate rows, while

UNION ALL includes them. When using these commands, rules such as data types, column order, and column alias uniqueness are essential.

It is important to note that

UNION can negatively impact performance due to the implied DISTINCT keyword, which sorts and removes duplicates. When working with larger datasets,

UNION ALL outperforms

UNION. Overall, understanding and effectively using these commands can help you manipulate data more efficiently.

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