How to Use Union in SQL Queries

Using UNION with SQL queries is a great way to explore the power of databases like MySQL and SQL servers. UNION allows you to combine results from different SELECT statements seamlessly. This simplified guide aims to make SQL UNION approachable and practical for users of all levels.

Also Read – Learn to Use Joins in SQL

Understanding UNION in SQL Query

In SQL, you will often find data scattered across different tables. SQL UNION bridges this gap, merging results from multiple SELECT statements. It unifies datasets effortlessly, harmonizing diverse data sources like employee info or sales figures across tables.

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Union Syntax for SQL Queries

SQL UNION is your go-to solution when you need to consolidate data from various sources. It’s like combining the best of both worlds, making your data manipulation tasks more efficient. The basic syntax is straightforward, enabling you to merge results effortlessly.

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

Let’s dive into examples and explore the capabilities of SQL UNION step by step.

Example: Union of Names in SQL Query

Imagine you have two tables, one for employees and another for contractors, both with a ‘name’ column. The need arises to create a unified list of names without duplicates. SQL UNION comes to the rescue.

-- Combine names from employees and contractors
SELECT name FROM employees
UNION
SELECT name FROM contractors;

UNION ALL for Duplicates

In some scenarios, you might want to include duplicates in your result set. For such cases, SQL UNION ALL provides a solution. It’s like saying, “Give me everything, even if it repeats.”

-- Include duplicates in combined names
SELECT name FROM employees
UNION ALL
SELECT name FROM contractors;

Order Results in SQL Query

Sorting your results can enhance readability. UNION in SQL query allows you to order your combined data. It’s like arranging your books alphabetically on a shelf for easy access.

-- Combine and order names alphabetically
SELECT name FROM employees
UNION
SELECT name FROM contractors
ORDER BY name;

Dealing with Different Column Counts

Sometimes, the SELECT statements may have a different number of columns. SQL UNION handles this gracefully. It’s like aligning pieces of a puzzle to create a complete picture.

-- Combine names and job titles
SELECT name, job_title FROM employees
UNION
SELECT name, '' AS job_title FROM contractors;

UNION in SQL Sub Queries

Subqueries add another layer of flexibility to SQL UNION. Instead of directly selecting from tables, you can use subqueries within each SELECT statement to perform more complex filtering or calculations.

-- Find employees and contractors with salaries above $50,000
SELECT name FROM employees WHERE salary > 50000
UNION
SELECT name FROM contractors WHERE hourly_rate * 40 * 52 > 50000;

In this example, subqueries (like WHERE salary > 50000) add detailed conditions, giving better control over combined data in SQL UNION for enhanced precision.

SQL UNION Tips to Use

Here are simplified tips for effective use of UNION in SQL queries:

Tip 1: Choose UNION ALL for Speed

If you’re okay with duplicates and want a faster query, use UNION ALL instead of UNION. It’s quicker but may give you more rows.

-- Combine data from emp and contract emp, including duplicates
SELECT emp_name FROM emp
UNION ALL
SELECT cont_name FROM cont_emp;

Tip 2: Check Data Types

Make sure the types of data (like numbers or text) match in your columns. Mismatched types can cause errors in your queries.

-- Mismatched data types may lead to errors
SELECT emp_id FROM emp
UNION
SELECT proj_name FROM proj; -- Error: different data types

Tip 3: Order Smartly with ORDER BY

Using ORDER BY in a SQL query with UNION ensures the final result is indeed sorted. However, if you need to sort each part differently, consider using subqueries.

-- Combine and order names alphabetically
SELECT emp_name FROM emp
UNION
SELECT cont_name FROM cont_emp
ORDER BY emp_name;

Tip 4: Test with Small Data First

Before using SQL UNION with big datasets, try it out with smaller sets first. It helps spot and fix issues faster.

-- Test with a small subset of data first
SELECT prod_name FROM electronics
UNION
SELECT prod_name FROM clothing
WHERE stock_qty > 10;

Tip 5: Explore Subqueries

Look into using SQL UNION within sub-queries for complex conditions and data consolidation. It’s handy for intricate logic.

-- Find employees and contractors with salaries above $50,000 using subqueries
SELECT emp_name FROM emp WHERE salary > 50000
UNION
SELECT cont_name FROM cont_emp WHERE hourly_rate * 40 * 52 > 50000;

By keeping these tips in mind, you can make the most of SQL UNION practically and straightforwardly.

Frequently Asked Questions (FAQs)

We have collected a bunch of questions related to the use of UNION in SQL queries. Hopefully, these will be helpful for you.

Q1: What is the purpose of using SQL UNION?

A1: SQL UNION combines results from different SELECT statements, providing a way to consolidate data from multiple tables into a single result set.

Q2: Can I use SQL UNION with tables having different columns?

A2: Yes, SQL UNION can be used with tables having different columns. Ensure the number of columns in each SELECT statement aligns.

Q3: Does SQL UNION remove duplicate rows by default?

A3: Yes, SQL UNION removes duplicate rows by default. If you want to include duplicates, use the UNION ALL operator.

Q4: Can I use ORDER BY with SQL UNION?

A4: Yes, you can use ORDER BY to sort the combined result set when using SQL UNION. It applies to the final result.

Q5: What happens if the data types don’t match between SELECT statements?

A5: SQL UNION requires the data types to match between corresponding columns. If they don’t, you may encounter errors.

Q6: Is there a performance impact when using SQL UNION?

A6: Using SQL UNION may impact performance, especially with large datasets. It’s advisable to test and optimize queries accordingly.

Q7: Can I use SQL UNION with more than two SELECT statements?

A7: Yes, SQL UNION can be used with more than two SELECT statements. You can chain multiple statements to combine data from several tables.

Q8: How does SQL UNION differ from UNION ALL?

A8: SQL UNION removes duplicates from the result set, while UNION ALL includes all rows, including duplicates, providing a faster but potentially larger result.

Q9: Can I use SQL UNION with subqueries?

A9: Yes, SQL UNION can be used with subqueries, allowing for more complex and versatile data consolidation in your queries.

Q10: Are there any limitations when using SQL UNION?

A10: One limitation is that the number of columns and their data types must match between corresponding SELECT statements. Additionally, ensure proper data alignment for meaningful results.

Summary – SQL Query UNION with Examples

By exploring SQL UNION through this simplified guide, you’ve unlocked a valuable skill for data consolidation. Whether you’re a beginner or an experienced user, mastering UNION elevates your SQL proficiency. Dive in, experiment, and witness the seamless merging of data in your SQL queries.

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