Summary: in this tutorial, you will learn how to select data from multiple tables by using the PostgreSQL INNER JOIN clause.
Introduction to PostgreSQL INNER JOIN clause
So far, you have learned how to select data from a table, choosing which columns and rows you want, and how to sort the result set in a particular order. It is time to move to one of the most important concepts in the database called joining that allows you to relate data in one table to the data in other tables. There are several kinds of joins including INNER JOIN, OUTER JOIN and self-join. This tutorial focuses on the INNER JOIN.
Suppose you want to get data from two tables named A and B. The B table has the fkafield that relates to the primary key of the Atable.
To get data from both tables, you use the INNER JOIN clause in the SELECT statement as follows:
1 2 3 4 5 6 7 8 | SELECT A.pka, A.c1, B.pkb, B.c2 FROM A INNER JOIN B ON A .pka = B.fka; |
To join A table to B table:
- First, you specify the column in both tables from which you want to select data in the
SELECTclause - Second, you specify the main table i.e., A in the
FROMclause. - Third, you specify the table that the main table joins to i.e., B in the
INNER JOINclause. In addition, you put a join condition after theONkeyword i.e, A.pka = B.fka.
For each row in the A table, PostgreSQL scans the B table to check if there is any row that matches the condition i.e., A.pka = B.fka. If it finds a match, it combines columns of both rows into one row and add the combined row to the returned result set.
The primary key column ( pka) and foreign key column ( fka) are typically indexed; therefore, PostgreSQL only has to check for the match in the indexes, which is very fast.
Sometimes A and B tables have the same column name so we have to refer to the column as table_name.column_name to avoid ambiguity. In case the name of the table is long, you can use a table alias e.g., tbland refer to the column as tbl.column_name.
The following Venn Diagram illustrates how PostgreSQL INNER JOIN clause works.
The INNER JOIN clause returns rows in A table that have the corresponding rows in the B table.
PostgreSQL INNER JOIN examples
PostgreSQL INNER JOIN to join 2 tables example
Let’s take a look at the customerand paymenttables in the sample database.
Each customer may have zero or many payments. Each payment belongs to one and only one customer. The customer_id field establishes the link between two tables.
You can use the INNER JOIN clause to join the customer table to payment table as follows:
1 2 3 4 5 6 7 8 9 10 | SELECT customer.customer_id, first_name, last_name, email, amount, payment_date FROM customer INNER JOIN payment ON payment.customer_id = customer.customer_id; |
You can add the ORDER BY clause to sort the result set by customer id as follows:
1 2 3 4 5 6 7 8 9 10 11 12 | SELECT customer.customer_id, first_name, last_name, email, amount, payment_date FROM customer INNER JOIN payment ON payment.customer_id = customer.customer_id ORDER BY customer.customer_id; |
You can also use a WHERE clause to filter customer. The following query returns customer’s rental data for the customer id 2:
1 2 3 4 5 6 7 8 9 10 11 12 | SELECT customer.customer_id, first_name, last_name, email, amount, payment_date FROM customer INNER JOIN payment ON payment.customer_id = customer.customer_id WHERE customer.customer_id = 2; |
PostgreSQL INNER JOIN to join 3 tables example
The following diagram illustrates the relationship between three tables: staff, payment, and customer.
- Each staff relates to zero or many payments. Each payment is processed by one and only one staff.
- Each customer has zero or many payments. Each payment belongs to one and only customer.
To join the three tables, you place the second INNER JOIN clause after the first INNER JOIN clause as the following query:
1 2 3 4 5 6 7 8 9 10 11 12 13 | SELECT customer.customer_id, customer.first_name customer_first_name, customer.last_name customer_last_name, customer.email, staff.first_name staff_first_name, staff.last_name staff_last_name, amount, payment_date FROM customer INNER JOIN payment ON payment.customer_id = customer.customer_id INNER JOIN staff ON payment.staff_id = staff.staff_id; |
To join more than three tables, you apply the same technique.
In this tutorial, we have shown you how to select data from multiple tables by joining one table to other tables using PostgreSQL INNER JOIN clause.