description: Learn how to use UNION, INTERSECT, and EXCEPT to combine results from multiple queries
order: 150
type: lesson-challenge
setup: |
```sql
CREATE TABLE fiction_books (
CREATE TABLE fiction_book (
id INT PRIMARY KEY,
title VARCHAR(255),
author VARCHAR(255),
price DECIMAL(10, 2)
);
CREATE TABLE non_fiction_books (
CREATE TABLE non_fiction_book (
id INT PRIMARY KEY,
title VARCHAR(255),
author VARCHAR(255),
price DECIMAL(10, 2)
);
INSERT INTO fiction_books (id, title, author, price)
INSERT INTO fiction_book (id, title, author, price)
VALUES
(1, 'The Great Gatsby', 'F. Scott Fitzgerald', 24.99),
(2, '1984', 'George Orwell', 19.99),
@ -27,7 +27,7 @@ setup: |
(4, 'The Hobbit', 'J.R.R. Tolkien', 29.99),
(5, 'The Doors of Perception', 'Aldous Huxley', 12.99);
INSERT INTO non_fiction_books (id, title, author, price)
INSERT INTO non_fiction_book (id, title, author, price)
VALUES
(1, 'A Brief History of Time', 'Stephen Hawking', 29.99),
(2, 'The Art of War', 'Sun Tzu', 19.99),
@ -52,11 +52,7 @@ setup: |
Sometimes you need to combine results from multiple queries into a single result set. SQL provides several set operations for this purpose: `UNION`, `UNION ALL`, `INTERSECT`, and `EXCEPT`.
## UNION and UNION ALL
`UNION` combines results from two or more queries and removes duplicates. `UNION ALL` does the same but keeps duplicates.
I have setup the following two tables `fiction_books` and `non_fiction_books` with the following data:
Let' take a look at some examples to understand how these set operations work. I have setup the following two tables `fiction_book` and `non_fiction_book` with the following data:
@ -74,16 +70,20 @@ I have setup the following two tables `fiction_books` and `non_fiction_books` wi
| 4 | Pride and Prejudice: A Study Guide | John Smith | 12.99 |
| 5 | The Doors of Perception | Aldous Huxley | 12.99 |
Let's start with the `UNION` operations.
`UNION` combines results from two or more queries and removes duplicates. `UNION ALL` does the same but keeps duplicates.
### UNION Example
Let's find all books across fiction and non-fiction categories:
Let's find all books across `fiction_book` and `non_fiction_book` tables using a `UNION` operation:
```sql
SELECT title, author, price
FROM fiction_books
FROM fiction_book
UNION
SELECT title, author, price
FROM non_fiction_books
FROM non_fiction_book
ORDER BY title;
```
@ -101,16 +101,16 @@ The query above will combine the results from both the queries and remove any du
| The Great Gatsby | F. Scott Fitzgerald | 24.99 |
| The Hobbit | J.R.R. Tolkien | 29.99 |
Notice how the `The Doors of Perception` book is only listed once even though it appears in both the `fiction_books` and `non_fiction_books` tables.
Notice how the `The Doors of Perception` book is only listed once even though it appears in both the `fiction_book` and `non_fiction_book` tables.
### UNION ALL Example
If you want to keep duplicates, use UNION ALL:
```sql
SELECT title, author, price FROM fiction_books
SELECT title, author, price FROM fiction_book
UNION ALL
SELECT title, author, price FROM non_fiction_books
SELECT title, author, price FROM non_fiction_book
ORDER BY title;
```
@ -133,12 +133,12 @@ Notice how the `The Doors of Perception` book is listed twice.
## INTERSECT
`INTERSECT` returns only the rows that appear in both result sets. Let's find books that are in both the `fiction_books` and `non_fiction_books` tables:
`INTERSECT` returns only the rows that appear in both result sets. Let's find books that are in both the `fiction_book` and `non_fiction_book` tables:
```sql
SELECT title, author FROM fiction_books
SELECT title, author FROM fiction_book
INTERSECT
SELECT title, author FROM non_fiction_books;
SELECT title, author FROM non_fiction_book;
```
This will return:
@ -149,12 +149,12 @@ This will return:
## EXCEPT
`EXCEPT` returns rows from the first query that don't appear in the second query. Let's find the books in the `fiction_books` table that aren't in the `non_fiction_books` table:
`EXCEPT` returns rows from the first query that don't appear in the second query. Let's find the books in the `fiction_book` table that aren't in the `non_fiction_book` table:
```sql
SELECT title, author FROM fiction_books
SELECT title, author FROM fiction_book
EXCEPT
SELECT title, author FROM non_fiction_books
SELECT title, author FROM non_fiction_book
ORDER BY title;
```
@ -177,24 +177,48 @@ For example, below is an invalid query:
```sql
-- ERROR: each UNION query must have the same number of columns
SELECT title, author FROM fiction_books
SELECT title, author FROM fiction_book
UNION
SELECT title, author, price FROM non_fiction_books;
SELECT title, author, price FROM non_fiction_book;
```
Because the `fiction_books` table has 2 columns and the `non_fiction_books` table has 3 columns, the query will return an error.
The above query will return an error because the `fiction_book` table has 2 columns and the `non_fiction_book` table has 3 columns.
Similary if we try to combine the following two queries:
```sql
-- ERROR: UNION types numeric and character varying cannot be matched
SELECT title, price FROM fiction_books
SELECT title, price FROM fiction_book
UNION
SELECT title, author FROM non_fiction_books;
SELECT title, author FROM non_fiction_book;
```
This will also return an error because the `price` column from the first query doesn't have the same data type as the `author` column from the second query.
Also, another important thing to note for the set operations is that the column names in the result set will be the column names from the first query. For example, if we run the following query:
```sql
SELECT title, author as 'writer_name' FROM fiction_book
Kamran Ahmed
3 weeks ago