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What are Joins in PostgreSQL?

PostgreSQL Joins are used for retrieving data from more than one tables. With Joins, it is possible for us to combine the SELECT and JOIN statements into a single statement. A JOIN condition is added to the statement, and all rows that meet the conditions are returned. The values from different tables are combined based on common columns. The common column mostly is a primary key in the first table and a foreign key of the second table.

In this PostgreSQL JOINs tutorial, you will learn:

PostgreSQL Join Types

There are two types of Joins in PostgreSQL:

Inner Joins

Outer Joins

PostgreSQL Inner Join

There are 3 types of Inner Joins in PostgreSQL:

Theta join

Natural join

EQUI join

Theta Join

A theta join allows one to join two tables based on the condition that is represented by theta. Theta joins can work with all comparison operators. In most cases, the theta join is referred to as inner join.

The theta join is the most basic type of JOIN. It will return all rows from the tables where the JOIN condition is satisfied.

Syntax:

SELECT columns FROM table-1 INNER JOIN table-2 ON table-1.column = table-2.column;

Consider the following tables of the Demo database:

Book:

Price:

We want to see the name of each book and the corresponding Price. We can run the following command:

SELECT Book.name, Price.price FROM Book INNER JOIN Price ON chúng tôi = chúng tôi

This will return the following:

Only 3 rows satisfied the join condition.

EQUI Join

The EQUI join provides us with a way of joining two tables based on primary key/foreign key relationship. For example:

SELECT * FROM Book JOIN Price ON chúng tôi = Price.id;

This will return the following:

Records have been returned from both tables based on the common columns, that is, the id column.

Natural Join

This type of join provides us with another way of writing an EQUI join. We can improve our previous example by adding the NATURAL keyword as shown below:

SELECT * FROM Book NATURAL JOIN Price;

This will return the following:

Only one id column has been returned. The NATURAL JOIN was able to note that the id column is common in the two tables. Only one was returned.

Postgres Outer Join

There are 3 types of Outer Joins in PostgreSQL:

Left Outer Join

Right Outer Join

Full Outer Join

Postgres Left Outer Join

The LEFT OUTER JOIN will return all rows in the table on the left-hand side and only the rows in the right-hand side table where the join condition has been satisfied.

Syntax:

SELECT columns FROM table-1 LEFT OUTER JOIN table-2 ON table-1.column = table-2.column;

We need to see the name of each book and the corresponding Price. We can run the following command:

SELECT Book.name, Price.price FROM Book LEFT JOIN Price ON chúng tôi = chúng tôi

This returns the following:

All the 4 rows in the Book table have been returned. Only 3 rows from the Price table met the join condition. Hence they were returned. The last book has no corresponding price value.

Postgres Right Outer Join

The RIGHT OUTER JOIN returns all rows in the table on the right-hand side and rows in the table on the left-hand side where the join condition has been satisfied.

Syntax:

SELECT columns FROM table-1 RIGHT OUTER JOIN table-2 ON table-1.column = table-2.column;

For example:

SELECT Book.name, Price.price FROM Book RIGHT JOIN Price ON chúng tôi = Price.id;

This returns the following:

All the rows in the Price table have been returned. Only the rows in the Book table that met the join condition were returned. The 3rd row has no value for name since no match was found.

Full Outer Join in PostgreSQL

This type of JOIN will return all rows in the table on the left-hand side and all rows in the table on the right-hand side with nulls where the join condition is not satisfied.

Syntax:

SELECT columns FROM table-1 FULL OUTER JOIN table-2 ON table-1.column = table-2.column;

For example:

SELECT Book.name, Price.price FROM Book FULL OUTER JOIN Price ON chúng tôi = Price.id;

This returns the following:

All rows from all tables have been returned, with nulls where no match was found.

Using pgAdmin

The above tasks can be accomplished in pgAdmin as follows:

pgAdmin Inner Join How To Use Theta Join in PostgreSQL using pgAdmin

Below are the steps to use Theta Join in Postgres using pgAdmin:

Step 1) Login to your pgAdmin account

Open pgAdmin and Login using your credentials

Step 2) Create Demo database

Step 3) Type the query

Type the below query in the query editor:

SELECT Book.name, Price.price FROM Book INNER JOIN Price ON chúng tôi = chúng tôi

Step 4) Execute the query