PostgreSQL INTERSECT Clause: Finding Common Elements in Result Sets

Introduction

You’ve got two queries and you want to know what they have in common. How do you go about it? Enter the INTERSECT clause in PostgreSQL. It helps you find rows that are common to two SELECT queries. Let’s jump right in and explore how you can use INTERSECT effectively, with tips to avoid common pitfalls and make the most of your queries.

Basic Syntax and Usage

The structure of an INTERSECT query is straightforward. It involves two or more SELECT queries separated by the INTERSECT keyword.

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

The queries should select the same number of columns with compatible data types.

Example with tv_series and web_series Tables

Say you want to find out which series names are common between your tv_series and web_series tables. Here’s how you can do it:

SELECT name FROM tv_series
INTERSECT
SELECT name FROM web_series;

Common Flags and Usage Patterns

• Column Compatibility: Make sure columns in both queries are compatible in terms of data types.

• Ordered Intersect: You can use ORDER BY to sort the final result set.

  SELECT name FROM tv_series WHERE release_year > 2015
  INTERSECT
  SELECT name FROM web_series WHERE release_year > 2015
  ORDER BY name ASC;

Common Errors and Solutions

Error Message:

ERROR:  each INTERSECT query must have the same number of columns

Cause:
The queries used with INTERSECT must have the same number of columns.

Solution:
Revise your queries to make sure both have the same number of columns with compatible data types.

Pro Tips

• INTERSECT removes duplicates from the final result set, similar to a SELECT DISTINCT operation.

• Use INTERSECT wisely. While it’s tempting to use it for complex queries, performance can take a hit with large data sets.

And there you have it! A comprehensive guide to using INTERSECT in PostgreSQL.