Unions in MySQL | Intermediate MySQL

Introduction

This tutorial will guide you through the concept of unions in MySQL, a powerful feature that allows you to combine results from multiple SELECT statements into a single result set. Understanding unions is essential for data analysts, as it enables the retrieval of data from multiple tables efficiently.

Step 1: Understanding Unions

Unions are used to combine the result sets of two or more SELECT statements. Here’s what you need to know:

• Each SELECT statement within a union must have the same number of columns.
• The columns must have similar data types.
• The column names in the result set are taken from the first SELECT statement.

Practical Tip

Use unions when you need to aggregate similar data from different tables, like sales data from different regions.

Step 2: Basic Syntax of Unions

The basic syntax for using a union in MySQL is:

SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2;

• Replace column1 and column2 with the actual column names you want to retrieve.
• Replace table1 and table2 with the names of your tables.

Common Pitfall

Ensure that the number and order of columns match across all SELECT statements. If they don't, you will encounter an error.

Step 3: Using UNION ALL

In some cases, you may want to include duplicate records. This is where UNION ALL comes in handy. The syntax is similar:

SELECT column1, column2 FROM table1
UNION ALL
SELECT column1, column2 FROM table2;

• UNION ALL returns all records, including duplicates.

Practical Advice

Use UNION ALL for performance benefits when you know there are no duplicates to worry about, as it is faster than a standard union.

Step 4: Sorting Results

You can sort the results of a union using the ORDER BY clause, but it should be applied at the end of the entire union query:

SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2
ORDER BY column1;

Important Note

The ORDER BY clause can only reference columns from the first SELECT statement in the union.

Step 5: Filtering with WHERE Clauses

You can also use WHERE clauses to filter results in each SELECT statement:

SELECT column1 FROM table1 WHERE condition1
UNION
SELECT column1 FROM table2 WHERE condition2;

Example Conditions

• condition1: status = 'active'
• condition2: date > '2023-01-01'

Conclusion

Unions in MySQL are a powerful way to combine data from multiple sources into a single result set. By understanding how to use both UNION and UNION ALL, as well as how to sort and filter your results, you can enhance your data retrieval processes. As a next step, practice writing your own union queries using different tables and conditions to solidify your understanding.