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INTERSECT

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Overview

The INTERSECT combines the result sets of two or more SELECT statements, retrieving only the common rows between them. Unlike UNION, which combines all rows and removes duplicates, INTERSECT focuses on returning rows that appear in all SELECT statements.

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Syntax

The syntax for the INTERSECT is as follows: 
SELECT value1, value2, ... value_n
FROM table1
INTERSECT
SELECT value1, value2, ... value_n
FROM table2;
The parameters from the syntax are explained below:
  • value1, value2, ... value_n: The columns you want to retrieve. You can also use SELECT * FROM to retrieve all columns.
  • table1, table2: The tables from which you wish to retrieve records.
The data types of corresponding columns must be compatible.

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Example

Suppose you have two tables: customers_old and customers_new, containing customer data for different periods. You want to find the customers who are present in both tables: 
CREATE TABLE customers_old (
    customer_id INT,
    customer_name TEXT
);

CREATE TABLE customers_new (
    customer_id INT,
    customer_name TEXT
);

INSERT INTO customers_old VALUES
(1, 'Alice'),
(2, 'Bob'),
(3, 'Charlie');

INSERT INTO customers_new VALUES
(2, 'Bob'),
(3, 'Charlie'),
(4, 'David');
Viewing the inserted values: 
SELECT * FROM customers_old;
SELECT * FROM customers_new;

customer_id | customer_name
-------------+---------------
           1 | Alice
           2 | Bob
           3 | Charlie

 customer_id | customer_name
-------------+---------------
           2 | Bob
           3 | Charlie
           4 | David
Now, let’s combine common customers using the INTERSECT: 
SELECT customer_name FROM customers_old
INTERSECT
SELECT customer_name FROM customers_new;
The result will include only the names that appear in both tables: 
customer_name
---------------
 Bob
 Charlie
The picture displays a list of customer names that appear in both tables. Only “Bob” and “Charlie” are found in both tables and shown as INTERSECT’s final result.

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INTERSECT ALL

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Overview

The INTERSECT ALL retrieves all common rows between two or more tables, including duplicates. This means that if a row appears multiple times in any of the SELECT statements, it will be included in the final result set multiple times.

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Syntax

The syntax for INTERSECT ALL is similar to INTERSECT: 
SELECT value1, value2, ... value_n
FROM tables
INTERSECT ALL
SELECT value1, value2, ... value_n
FROM tables;
The parameters from the syntax are explained below:
  • value1, value2, ... value_n: The columns you wish to retrieve. You can also retrieve all the values using the SELECT * FROM query.
  • table1, table2: The tables from which you want to retrieve records.
The data types of corresponding columns in the SELECT queries must be compatible.

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Example

Let’s create three tables of products from different years. You want to find the common products among all three categories, including duplicates. 
CREATE TABLE products_electronics2021 (
    product_id INT,
    product_name TEXT
);

CREATE TABLE products_electronics2022 (
    product_id INT,
    product_name TEXT
);

CREATE TABLE products_electronics2023 (
    product_id INT,
    product_name TEXT
);

INSERT INTO products_electronics2021 VALUES
(1, 'Laptop'),
(2, 'Phone'),
(3, 'Tablet'),
(4, 'Headphones');

INSERT INTO products_electronics2022 VALUES
(2, 'TV'),
(3, 'Printer'),
(4, 'Monitor'),
(5, 'Phone');

INSERT INTO products_electronics2023 VALUES
(3, 'Laptop'),
(4, 'Phone'),
(5, 'Oven'),
(6, 'AC');
Display the tables using the query below: 
SELECT * FROM products_electronics2021;
SELECT * FROM products_electronics2022;
SELECT * FROM products_electronics2023;

product_id | product_name
------------+--------------
          1 | Laptop
          2 | Phone
          3 | Tablet
          4 | Headphones

 product_id | product_name
------------+--------------
          2 | TV
          3 | Printer
          4 | Monitor
          5 | Phone

 product_id | product_name
------------+--------------
          3 | Laptop
          4 | Phone
          5 | Oven
          6 | AC
Then, combine common products from all three categories using the INTERSECT ALL: 
SELECT product_name FROM products_electronics2021
INTERSECT ALL
SELECT product_name FROM products_electronics2022
INTERSECT ALL
SELECT product_name FROM products_electronics2023;
The result will include the products that are common among all three categories, including duplicates: 
product_name
--------------
 Phone
The illustration shows a list of product names common to all three years, including duplicates. In this case, the result is the product name “Phone,” which appears across all three tables.