PostgreSQL: INTERSECT Operator

In this PostgreSQL post explains how to use the INTERSECT operator with syntax and examples.

Description

The PostgreSQL INTERSECT operator returns the intersection of 2 or more datasets. Each dataset is defined by a SELECT statement. If a record exists in both data sets, it will be included in the INTERSECT results. However, if a record exists in one data set and not in the other, it will be omitted from the INTERSECT results.

Intersect Query

Explanation: The INTERSECT query will return the records in the blue shaded area. These are the records that exist in both Dataset1 and Dataset2.

Each SELECT statement within the INTERSECT must have the same number of fields in the result sets with similar data types.

Syntax

The syntax for the INTERSECT operator in PostgreSQL is:

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
INTERSECT
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

Parameters or Arguments

expression1, expression2, expression_n

The columns or calculations that you wish to retrieve.

tables

The tables that you wish to retrieve records from. There must be at least one table listed in the FROM clause.

WHERE conditions

Optional. These are conditions that must be met for the records to be selected.

Note

• There must be same number of expressions in both SELECT statements.
• The corresponding expressions must have the same data type in the SELECT statements. For example: expression1 must be the same data type in both the first and second SELECT statement.

Example - With Single Expression

The following is an INTERSECT operator example that has one field with the same data type:

SELECT category_id
FROM products
INTERSECT
SELECT category_id
FROM inventory;

In this INTERSECT example, if a category_id appeared in both the products and inventory table, it would appear in your result set.

Now, let's complicate our example further by adding WHERE conditions to the INTERSECT query.

SELECT category_id
FROM products
WHERE category_id  800
INTERSECT
SELECT category_id
FROM inventory
WHERE quantity > 5;

In this example, the WHERE clauses have been added to each of the datasets. The first dataset has been filtered so that only records from the products table where the category_id is less than 800 are returned. The second dataset has been filtered so that only records from the inventory table are returned where the quantity is greater than 5.

Example - With Multiple Expressions

Next, let's look at an example of how to use the INTERSECT operator in PostgreSQL to return more than one column.

For example:

SELECT contact_id, last_name, first_name
FROM contacts
WHERE last_name > 'Smith'
INTERSECT
SELECT customer_id, last_name, first_name
FROM customers
WHERE customer_id  100;

In this INTERSECT example, the query will return the records from the contacts table where the contact_id, last_name, and first_name values match the customer_id, last_name, and first_name value from the customers table.

There are WHERE conditions on each data set to further filter the results so that only records from the contacts are returned where the last_name is not Smith. The records from the customers table are returned where the customer_id is less than 100.

Example - Using ORDER BY

The following is an INTERSECT example that uses a ORDER BY clause:

SELECT contact_id, contact_name
FROM contacts
WHERE contact_id  100
INTERSECT
SELECT company_id, company_name
FROM companies
WHERE state = 'CA'
ORDER BY 1;

Since the column names are different between the two SELECT statements, it is more advantageous to reference the columns in the ORDER BY clause by their position in the result set. In this example, we've sorted the results by contact_id / company_id in ascending order, as denoted by the ORDER BY 1.

The contact_id / company_id fields are in position #1 in the result set.