How to use Union Clause in SQL

When it comes to working with large databases, it’s important that we utilize the appropriate SQL clauses available in order to accomplish our goals in the shortest possible time frame. SQL Union is one such clause that can help us combine the results of multiple SELECT queries into a single result set. In this article, we will discuss the basics of UNION clauses and some common use cases.

What is an SQL Union Clause?

A Union clause is a SQL Operator used to combine the results of multiple SELECT statements into a single result, which is then returned as a single, table-like structure. The individual SELECT statements must have the same number of columns, with corresponding data types, and must appear in the same order. SQL Union is used to join rows of two or more tables into a single result set. NULL values are also included in the result.

How Does It Work?

The Union clause in SQL combines the rows that are retrieved by two or more SELECT statements into a single result set. The order of the columns in each SELECT statement must match. The Union clause also requires that the column data types in each SELECT statement match. If they don’t, you will receive an error message.

When two or more SELECT statements are combined using Union, duplicates are removed from the result set. If we want to keep the duplicates, we can use the Union All clause.

Here’s an example of how a Union statement works:

SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2

The above query will return the results in the following order:

1. All the rows returned by the first SELECT statement
2. All the rows returned by the second SELECT statement
3. Duplicate rows are removed from the result set (unless we are using Union All).

Union clauses can also be used to join multiple tables based on specific conditions.

Common Use Cases of Union Clauses

Combine two tables with the same structure: Let’s say you have two tables with the same schema and you want to combine them into a single result set. You can use SQL Union to achieve this.

SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2

Combine multiple results into a single result set: Let’s say you have multiple queries that return different parts of the information you need, and you want to combine all their results.

SELECT column_name(s) FROM table1 WHERE condition
UNION
SELECT column_name(s) FROM table2 WHERE condition
UNION
SELECT column_name(s) FROM table3 WHERE condition

Merge data from two tables with different structures: If you have two tables with different structures, but each contains important data and you want to merge them, you can use Union in combination with extra columns that normalize the output.

SELECT column1, column2, '' FROM table1
UNION
SELECT '' as column1, column2, column3 FROM table2

Frequently Asked Questions

Conclusion

SQL Union is a powerful operator that helps combine large amounts of data into a single result set. It is a useful tool that helps to retrieve relevant information from multiple tables and can be used in various applications. By adhering to the guidelines and best practices mentioned in this article, you can perform your database operations more efficiently.

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