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.