UNION & UNION ALL

Introduction  

The UNION operation is one of the most widely used operators in SQL. It is used to combine the results of two or more SELECT statements into a single result set. In SQL Server Management Studio (SSMS), UNION is a powerful tool that allows for data manipulation in a variety of ways. This blog will provide an overview of the advantages and disadvantages of using UNION in SSMS and provide easy to complex examples of how to use it. 

Background Information 

The UNION operation is used to combine the results of two or more SELECT statements into a single result set. The result set of the UNION operation contains only distinct rows that appear in either of the SELECT statements. The UNION operation is commonly used in SQL to combine data from different tables or views. 

Key Concepts and Terminology 

There are several key concepts and terminology related to the UNION operation, including: 

  • SELECT Statement: A SELECT statement is a SQL statement used to retrieve data from a database. 
  • UNION Operator: The UNION operator is used to combine the results of two or more SELECT statements into a single result set. 
  • UNION ALL Operator: The UNION ALL operator is used to combine the results of two or more SELECT statements into a single result set, including duplicate rows. 
  • Result Set: A result set is the set of rows returned by a SQL statement. 

Current Trends and Developments  

The UNION operation has been a staple in SQL for many years and continues to be a commonly used operation in database management. However, recent developments in big data and NoSQL databases have led to the emergence of new data management techniques that may eventually replace the UNION operation in some cases. 

Advantages of UNION in SSMS: 

  • The UNION operation allows for the combination of data from multiple tables or views into a single result set. 
  • The UNION operation removes any duplicate rows from the result set. 
  • The UNION operation is relatively easy to use and understand. 
  • Combine data from multiple tables: With the UNION operation, you can easily combine data from two or more tables into a single result set. This is useful when you want to analyze data from different tables that have a common column. 
  • Remove duplicate rows: When you use UNION, the result set contains only distinct rows. This is helpful when you need to eliminate duplicates from a large data set. 
  • Simplify complex queries: By using UNION, you can simplify complex queries that involve multiple joins and subqueries. This can make your queries easier to read and understand. 
  • Easy to use: The UNION operation is easy to use and understand. You only need to specify the columns you want to select and the tables you want to combine. 

Disadvantages of UNION in SSMS: 

  • The UNION operation can be slow when working with large amounts of data. 
  • The UNION operation may not always return the desired results if the SELECT statements are not properly constructed. 
  • Requires matching columns: The tables you want to combine with UNION must have the same number of columns and matching data types. If they do not, you may need to use the UNION ALL operation.

 Easy to Complex Examples of UNION in SSMS: 

  1. Combining data from two tables: 

This SQL statement will combine the data from table1 and table2, removing any duplicate rows. 

Combining data from three tables: 

This SQL statement will combine the data from table1, table2, and table3, removing any duplicate rows. 

  1. Combining data from two tables with different columns: 

This SQL statement will combine the data from table1 and table2, but only the columns that match will be included in the result set. 

  1. Combining data from two tables with different data types: 

This SQL statement will combine the data from table1 and table2, but the CAST function is used to convert the data type of column3 to match the data type of column2.

Case Studies/Examples: 

Here are some examples of how the UNION operation can be used in SQL: 

  • Example 1: Combine data from two tables 

 

This SQL statement will combine the data from table1 and table2, removing any duplicate rows. 

Example 2: Combine data from multiple tables 

This SQL statement will combine the data from table1, table2, and table3, removing any duplicate rows. 

Conclusion:  

  • The UNION operation is a powerful tool in SSMS that allows for the combination of data from multiple tables or views into a single result set. While the UNION operation has some disadvantages, such as slow performance with large datasets, its advantages make it a widely used tool for data manipulation in SQL. By using the easy to complex examples provided, you can use UNION to combine data from multiple tables and simplify complex queries. 
  • Challenges and Opportunities: One challenge with the UNION operation is its performance when working with large datasets. As big data and NoSQL databases continue to gain popularity, alternative data management techniques may eventually replace the UNION operation in some cases. However, the UNION operation remains a powerful tool for combining data from multiple tables or views in SQL. 
  • The UNION operation is a commonly used operation in SQL for combining data from multiple tables or views into a single result set. While the UNION operation may not always be the most efficient way to manage data, it remains a powerful tool for data manipulation in SQL.