Master Union with 3 Tables SQL Query Optimization

In the realm of SQL, a structured query language, the ability to combine and manipulate data from multiple tables is not just desirable but often essential. This is where joining tables with UNION comes into play. Let's dive into the intricacies of UNION clauses with SQL, specifically using UNION with three tables.

Before we proceed to using UNION with three tables, it's crucial to understand its fundamentals. UNION is used to combine the result-set of two or more SELECT statements. Each SELECT statement within UNION must have the same number of columns and the same data type for each column. It's like merging rows from different tables that have a similar structure.

Building Blocks of UNION with Three Tables

The basic syntax for UNION is as follows:

three vennuous circles with the words right table, left table and right table

SELECT statement UNION SELECT statement UNION SELECT statement;

Understanding the SELECT Statements

What is the difference between UNION and UNION ALL in SQL?

In this context, each SELECT statement represents a query that retrieves data from a table. For example, you might have:

SELECT column_name FROM table1;

SELECT column_name FROM table2;

Difference between UNION vs UNION ALL in SQL? Example

SELECT column_name FROM table3;

Arranging the SELECT Statements in UNION

The key to successful usage lies in arrange these SELECT statements in UNION. For instance, you could have:

SELECT column_name FROM table1 UNION SELECT column_name FROM table2 UNION SELECT column_name FROM table3;

This arrangement would combine the output of each SELECT statement into a single result set.

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Applying UNION with Three Tables

The concept remains the same when applying UNION with three tables. Let's consider we have three tables - 'Employee', 'InternalClient', and 'ExternalClient', and we want to retrieve specific data from all three. Here's how you might structure your SQL:

SELECT employee_id, employee_name FROM Employee UNION SELECT client_id, client_name FROM InternalClient UNION SELECT client_id, client_name FROM ExternalClient;

In this example, we're selecting 'employee_id' and 'employee_name' from the 'Employee' table, then the same columns (for consistency) from the 'InternalClient' and 'ExternalClient' tables. The resulting output will include all entries from the 'employee_id' and 'employee_name' fields across all three tables.

Handling Duplicate Values in UNION

By default, UNION eliminates duplicate rows. This is a key characteristic that makes it useful for combining results. However, if required, you can use UNION ALL to preserve duplicated values.

Adding a WHERE Clause to Filter Results

If you want to filter results from your UNION statement, you can add a WHERE clause to each SELECT statement. This allows for precise control over the data you're selecting.

Union with three tables in SQL can be a powerful tool when you need to combine data from multiple sources. Whether it's for creating a more consolidated report or preparing a dataset for further analysis, it's a very useful feature. The key is understanding its fundamentals, structuring your SELECT statements carefully, and using it wisely based on your needs.