SQL JOINS
Is a clause that seamlessly merges records from multiple tables within a database by utilizing common values in fields.
When working with databases, using SQL JOINs is an inevitable part of the process. This guide offers a concise yet clear overview of SQL JOINs and introduces you to the most commonly used types of JOINs
A Closer Look at SQL Join Definitions and Uses
Let me explain what a database is in simple terms. It is a group of tables that hold different kinds of information. To fetch data from related tables in a database, we use the JOIN clause. The SQL JOIN clause is a bit more complicated than a basic query that extracts data from a single table because it fetches data from multiple tables at once.
Exploring the Different Types of SQL Joins with Examples
It's essential to know that there are four types of SQL JOINs that you can use depending on the results you want to achieve. These are Inner JOIN, Left Outer JOIN, Right Outer JOIN, and Full Outer JOIN. It's crucial to understand that each type works differently, and you can use various sample SQL JOIN clauses as examples. However, keep in mind that you may come across other examples on different sites that simply remove the keyword "Outer".
Inner Join
Joining tables is a way to combine data from two different tables based on a shared key. For instance, you could join a table that contains a column for "product_id" to another table with the same column to find all the information associated with a particular user.
One way to do this is by using an Inner Join clause in your SQL query.
Here's an example:
SELECT * FROM products INNER JOIN product_details ON products.id = product_details.product_id;
This query will select all the columns from both tables where the "id" column in products matches the "product_id" column in product_detail.
Left Outer Join
When using a Left Outer Join all rows from the first table are returned along with only the matching rows from the second table.
One way to do this is by using a Left Outer Join clause in your SQL query.
Here's an example:
SELECT * FROM customers LEFT OUTER JOIN orders ON customers.id = orders.customer_id
This query will select all the columns from both tables where the "id" column in customers matches the "customer_id" column in orders. More specifically, the query would tell all the customers that have placed orders.
Right Outer Join
A Right Outer Join is a type of JOIN that returns all rows from the second table and only the matching rows from the first table. They are the opposite of a Left Outer Join, which returns all rows from the first table and only the matching rows from the second table.
One way to do this is by using a Right Outer Join clause in your SQL query.
Here's an example:
SELECT * FROM customers RIGHT OUTER JOIN orders ON customers.id = orders.customer_id
This query will select all the columns from both tables where the "id" column in customers matches the "customer_id" column in orders. More specifically, the query would tell all the customers that have not placed orders.
Full Outer Join
A Full JOIN is the combination of both left and right joins, which returns all rows from both tables, ensuring that there is at least one match between them. This type of join is also known as a Full Outer JOIN. This type of JOIN is also referred to as a Cartesian Product. In this example, we will use a Full Outer JOIN clause to join two tables, which is an effective way of merging data from multiple tables and obtaining comprehensive results.
SELECT * FROM customers FULL OUTER JOIN orders ON customers.id = orders.customer_id
This query will select all the columns from both tables where the "id" column in customers matches the "customer_id" column in orders. More specifically, the query would give a combination of all the customers and products.
When working with databases, SQL JOINs are essential for mapping out table relationships.
Real World Examples of using SQL JOINs
HR Application
Consider a scenario where there are two tables. One table stores user information like name, address, and phone number, while another table stores information related to security role of user. In the user table, each row represents a single user. Therefore, it would be more practical to store the security role data of the users in a separate table. This is because a user may appear multiple times in the security role table, with one row per security role as they may have multiple roles.
Suppose you are required to develop an application that displays the information of users, including their names, addresses, security roles, and joined dates. To fetch this data from the database, you would need to join two tables by using certain attributes shared between them (such as User ID).
Online Retail Store
Suppose you own an online store and wish to find out which products were purchased by your customers. To achieve this, you would have two tables - one with information about your customers and the other with information about your products. You can use an Inner JOIN to retrieve all the records that appear in both these tables.
Teacher Grading Application
Consider this scenario where you have two database tables: “Students” and “Grades”. The first table, “Students”, contains a single record for each student, including their ID number, name, major, and other relevant information. The second table, “Grades”, contains a single record of each student's grade in various courses, including their student ID number, the course they took, and their grade in that course.
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