Master SQL UNION: Combine Tables with Ease

Unlock the Power of SQL UNION: A Comprehensive Guide

What is SQL UNION?

When working with multiple tables, SQL UNION is a powerful operator that allows you to select fields from two or more tables. Imagine having two tables, Teachers and Students, and wanting to combine the name columns from both tables into a single result set. That’s exactly what SQL UNION does!

The Syntax

The SQL UNION syntax is straightforward:

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

Here, column1, column2, etc. are the column names required for the union, and table1 and table2 are the names of the tables to fetch the columns from.

How it Works

When using SQL UNION, remember these essential rules:

• The column count in all tables must be the same.
• The data type of columns must be the same.
• The columns must be in the same order in each table.

If these conditions are met, SQL UNION will combine the columns in the tables, ignoring any duplicate fields.

Example: SQL UNION in Action

Let’s say we want to retrieve the age columns from both the Teachers and Students tables, ignoring any duplicate values. Here’s the SQL command:

SELECT age
FROM Teachers
UNION
SELECT age
FROM Students;

This will return a result set with unique age values from both tables.

Adding Conditions with WHERE Clause

What if we want to select only the ages greater than or equal to 20 from both tables? We can use the WHERE clause with SQL UNION:

SELECT age
FROM Teachers
WHERE age >= 20
UNION
SELECT age
FROM Students
WHERE age >= 20;

This will return a result set with unique age values from both tables, meeting the specified condition.

SQL UNION ALL: The Difference

SQL UNION ALL is similar to SQL UNION, but with one key difference: it includes duplicate fields in the result set. Let’s try the previous example using UNION ALL instead of UNION:

SELECT age
FROM Teachers
WHERE age >= 20
UNION ALL
SELECT age
FROM Students
WHERE age >= 20;

This will return a result set with all age values from both tables, including duplicates.

Take Your SQL Skills to the Next Level

Want to learn more about SQL UNION and other powerful operators? Check out our guides on SQL JOIN, SQL SELECT, and SQL GROUP BY.