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
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
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
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.