UNION vs UNION ALL in SQL Server

Introduction

The results of numerous SELECT statements can be concatenated using the SQL UNION and UNION ALL operators. They differ from one another, nevertheless. In SQL, one of the main distinctions between the UNION and UNION ALL commands is that the former eliminates duplicates from the final results set, while the latter permits duplicates in the results set.

We shall examine the distinction between SQL's UNION and UNION ALL in this section.

What Separates UNION from UNION ALL?

Although both UNION and UNION ALL can be used to combine the output of two or more SELECT Statements, they differ greatly from one another.

Description	UNION	UNION ALL
Duplication Handling	The UNION presents only unique records by eliminating duplicate rows from the result set.	The UNION ALL retains every row—duplicates included—without any removal.
Performance Impact	The UNION adds an extra step of finding and getting rid of duplicates, which could affect performance.	Because the UNION ALL does not incur the overhead of duplicate removal, it typically operates more quickly.
Result Set Structure	Using unique records, the UNION generates a unique result set.	A result set containing every row from the combined queries is produced by the UNION ALL.
Syntax	The operator that is selected is the primary distinction between the syntaxes for UNION and UNION ALL.	The operator that is selected is the primary distinction between the syntaxes for UNION and UNION ALL.
Use Case	When removing duplicates is crucial, and focus is placed on unique records, the UNION is appropriate.	When keeping all rows—including duplicates—is acceptable or even desired, the UNION ALL is the better option.
Data Volume	UNION may not be as effective for huge datasets because of its intricate processing and redundant checks.	UNION ALL may be more effective for large datasets because of its streamlined processing and lack of duplicate checks.
Query Optimization	In order to find and remove duplicates, the UNION can require extra processing, which could extend its execution time.	Because the UNION ALL has fewer restrictions, the query may execute more quickly.
Resource Utilization	Memory usage is impacted by the UNION's additional resource consumption in locating and eliminating duplicate rows.	Since the UNION ALL omits the duplicate elimination step, it is typically more resource-efficient.
Compatibility	A wide range of relational database management systems (RDBMS) support UNION and UNION ALL.	A wide range of relational database management systems (RDBMS) support UNION and UNION ALL.
Consideration for Result Accuracy	UNION is the best option if removing duplicates is essential to the accuracy of the result set.	UNION ALL is their commended choice when performance is a top priority, and duplicate records are acceptable or required.

SQL UNION Operator

The set of one or more SELECT statements can be combined into a single result by using the SQL UNION Operator. Duplicate entries from the set of SELECT statements are eliminated from the combined result by the UNION operator.

A UNION statement requires that the data types be compatible and that the columns in the SELECT statement be in the same order. It's critical that different SELECT statements have the same amount of columns and data types.

Syntax

SELECT column1, column2, [column_n]
FROM table1
UNION
SELECT column1, column2, [column_n]
FROM table2;

UNION SQL Script

SELECT [Name], Country, Age, Mno 
FROM Employee1
UNION
SELECT [Name], Country, Age, Mno 
FROM Employee1;

UNION ALL Operator

Combining the result of one or more select statements as a set is also possible using the SQL UNION ALL operator. The distinction between UNION and UNION ALL lies in the fact that the former has duplicate values in the result sets of SELECT statements, while the latter does not. Because there isn't an extra step in UNION ALL to remove duplicates, it operates more quickly than the UNION statement.

Syntax

SELECT column1, column2, [column_n]
FROM table1
UNION ALL
SELECT column1, column2, [column_n]
FROM table2;

UNION ALL SQL Script

SELECT [Name], Country, Age, Mno 
FROM Employee1
UNION ALL
SELECT [Name], Country, Age, Mno 
FROM Employee1;

Summary

The distinctions between UNION and UNION ALL Operator were discussed in this article, along with some examples. Duplicate rows are eliminated from the result set by the UNION, but all rows—duplicates included remain in the UNION ALL result set after elimination. Furthermore, a wide range of relational database management systems (RDBMS) support UNION and UNION ALL.

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Happy coding!