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Mastering SQL Server UNION: Examples and Differences Explained

Are you a SQL Server user looking to improve your querying skills? If so, you may have heard of the UNION operator in SQL Server and how it can be used to combine the results of multiple SELECT statements into a single result set.

In this article, we will dive deeper into the topic of SQL Server UNION, including how it differs from JOIN and how to use it effectively in your queries.to SQL Server UNION

The SQL Server UNION operator is used to combine the results of two or more SELECT statements into a single result set. This means that the results of each SELECT statement are combined, and any duplicates are removed.

The UNION operator is often used when querying data from multiple tables or databases.

Query Requirements for SQL Server UNION

To use the UNION operator in your queries, there are a few requirements that must be met. First, all SELECT statements must include the same number of columns, and the data types of each corresponding column must be compatible.

Second, the columns in each SELECT statement must be in the same order. Finally, the ORDER BY clause can only be included in the last SELECT statement.

Venn Diagram Illustrating the Result Set

To better understand the result set produced by the UNION operator, it can be helpful to visualize it with a Venn diagram. Imagine two circles representing the result sets of two SELECT statements.

The area where the circles overlap represents the duplicates that will be removed by the UNION operator. The remaining areas of the circles represent the unique values from each SELECT statement.

When combined, the result is a single result set with all unique values from both SELECT statements. UNION vs.

UNION ALL

There is a key difference between the UNION operator and the UNION ALL operator in SQL Server. While the UNION operator removes duplicates from the result set, the UNION ALL operator does not.

This means that the UNION ALL operator returns all rows from each SELECT statement, including any duplicates. The decision to use UNION or UNION ALL depends on the specific requirements of the query.

UNION vs. JOIN

It is important to note that the UNION operator is not the same as the JOIN operator in SQL Server.

The UNION operator combines the results of multiple SELECT statements into a single result set, while the JOIN operator combines data from two tables based on a common column. While both operators are used to retrieve data from multiple sources, they serve different purposes and should be used according to specific requirements.

Difference Between JOIN and UNION

The main difference between JOIN and UNION is that JOIN combines data from two tables into a single table, while UNION combines the results of two or more SELECT statements into a single result set. JOIN is used when the data being combined has a common column, while UNION can be used to combine any SELECT statements with the same number and types of columns.

Illustration of the Difference between UNION and JOIN

To better understand the difference between UNION and JOIN, consider the following scenario. You have two tables: a customer table and an orders table.

The customer table contains information about the customers, while the orders table contains information about the orders placed by those customers. If you wanted to retrieve information about each customer and their orders, you would use a JOIN statement to combine the data from both tables.

However, if you wanted to retrieve a list of all customers as well as a list of all orders, you would use a UNION statement to combine the two SELECT statements.

Conclusion

In conclusion, SQL Server UNION is a powerful tool that can be used to combine the results of two or more SELECT statements into a single result set. It is important to keep in mind the requirements for using the UNION operator, as well as the differences between UNION and JOIN.

Understanding these concepts can help you to write more effective and efficient queries in SQL Server. SQL Server is a powerful database management system utilized by various organizations for storing and managing their data effectively.

One of the most frequently used operators in SQL Server is the UNION operator. In this article, we will discuss various SQL Server UNION examples to help you understand the concept better.

Overview of Staffs and Customers Tables

Consider the following database schema that represents the staffs and customers table in a company:

Staffs Table

Staff_id Staff_name Age Salary

1 John Doe 32 45000

2 Susan Smith 26 38000

3 Michael Lee 34 51000

Customers Table

Customer_id Customer_name Order_id

101 Alex Brown 1

102 Tom Grey 2

103 Henry Davis 3

UNION and UNION ALL Examples

Example 1: Suppose you need to retrieve a list of staff and customers from the tables, you could write the following query:

SELECT Staff_name AS Name, Age AS Age, Salary AS “Annual Income”, NULL AS “Order ID”

FROM Staffs

UNION

SELECT Customer_name, NULL AS Age, NULL AS “Annual Income”, Order_id AS “Order ID”

FROM Customers;

Here, we use the UNION operator to merge the employee and customer data, where the column Staff_name is returned as Name, Age from employees is returned as Age , Salary from employees is returned as Annual Income, and the Order_id from customers is returned as Order ID. Note that we use NULL values to match the number of columns in both tables.

Example 2: To retrieve all the data from both tables, including duplicates, use the UNION ALL operator:

SELECT Staff_name AS Name, Age AS Age, Salary AS “Annual Income”, NULL AS “Order ID”

FROM Staffs

UNION ALL

SELECT Customer_name, NULL AS Age, NULL AS “Annual Income”, Order_id AS “Order ID”

FROM Customers;

Result Set Comparison of Staffs and Customers Tables

The result set of the above examples looks like this:

Name Age Annual Income Order ID

John Doe 32 45000 NULL

Michael Lee 34 51000 NULL

Susan Smith 26 38000 NULL

Alex Brown NULL NULL 1

Tom Grey NULL NULL 2

Henry Davis NULL NULL 3

Notice that the columns with NULL values represent data that is not present in that specific table.

UNION and ORDER BY Example

You can use the ORDER BY clause with the UNION operator as well, though it is slightly more complex. The ORDER BY clause in a UNION query sorts the result set as a whole based on the columns specified.

Example: Suppose you want to retrieve a list of all names from the staff and customer tables, ordered alphabetically. Here is the query to achieve that:

SELECT Staff_name AS Name

FROM Staffs

UNION

SELECT Customer_name

FROM Customers

ORDER BY Name;

This query retrieves all names from the Staffs and Customers table and sorts them in alphabetical order. Note that we only retrieve the Name column and cast the Employee_Name and Customer_Name to the same column name to ensure consistency.

The result set would look like:

Name

Alex Brown

Henry Davis

John Doe

Michael Lee

Susan Smith

Tom Grey

Conclusion

SQL Server UNION operator is a powerful tool that enables you to combine data from multiple tables and manipulate the result set as needed. The examples we have discussed here provide a good foundation for using the UNION operator effectively.

With the knowledge of UNION examples, you can now write complex queries to retrieve the needed data from your SQL Server databases. In summary, SQL Server UNION operator is a significant tool for combining data from multiple tables and manipulating the result set in various ways.

By understanding its requirements, differences from other operators, and through numerous examples, you can enhance your querying skills while developing efficient and effective SQL Server queries. Takeaways from this article include:

– The UNION operator combines data from multiple SELECT statements into a single result set, removing duplicates and following query requirements.

– The UNION ALL operator combines all data from SELECT statements, including duplicates. – UNION and JOIN operators serve different purposes.

JOIN combines data from two tables based on a common column, while UNION combines SELECT statements with the same number and types of columns. – You can use the ORDER BY clause with the UNION operator to sort the result set for the columns specified.

By applying this knowledge, you can improve your SQL Server querying skills and make the most out of your databases.

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