Hello dear reader, today we will delve into the famous JOIN operations in SQL. You know, those magical touches that connect tables in a database, as if assembling a puzzle by joining pieces. We will talk about the intricacies of INNER, LEFT, RIGHT, and FULL JOINs, when to use them, and even why sometimes not to use them. Honestly, when I first learned this topic, it felt like learning a new language. But over time, it all made sense and settled in. Let’s see how simple it actually is:
JOIN Operations: INNER, LEFT, RIGHT, FULL
Now, suppose we have two tables. One is called ‘Customers’, and the other is ‘Orders’. The ‘Customers’ table contains each customer’s information such as ID, name, and surname. The ‘Orders’ table records which product each customer purchased and when. Naturally, when we want to see all orders placed by a customer or identify customers who haven’t ordered anything, these JOINs come into play.
Let’s start with the most well-known one: INNER JOIN. This type gives us exactly what we want. It retrieves records that exist in both the ‘Customers’ table and the ‘Orders’ table. Imagine wanting to see only customers who have placed orders. INNER JOIN does precisely that. If a customer has never ordered anything, that customer will not appear in the INNER JOIN result because their record doesn’t exist in both tables.
So, what if we want to see customers who haven’t placed any orders? That’s where LEFT JOIN comes in. When you use LEFT JOIN, it retrieves all records from the left table (in our example, ‘Customers’). The matching records from the right table (‘Orders’) are also included, but for customers without orders, the right table columns will be NULL. This helps us identify customers who haven’t ordered anything.