Как использовать INNER JOIN с WHERE в SQL?

🔎 When using the INNER JOIN clause in SQL, you can specify a condition to join two tables based on a common column or expression. The result of the join will include only the rows where the condition is true.

For example, let's say we have two tables, Table A and Table B, and we want to join them based on a common column named column C:

SELECT *
FROM TableA
INNER JOIN TableB ON TableA.C = TableB.C;
    

This query will return all the rows from Table A and Table B where the values in column C are the same in both tables.

It's important to note that the INNER JOIN only returns the matched rows between the two tables. If there are any unmatched rows, they will not be included in the result.

💡 Remember, the INNER JOIN clause is commonly used to combine related data from multiple tables in a database.

Understanding INNER JOIN in SQL

When working with databases, one of the most common operations is to retrieve data from multiple tables based on some relationship between them. This is where JOIN comes into play. Specifically, the INNER JOIN is used to combine rows from two or more tables based on a related column between them.

Syntax

The syntax for an INNER JOIN in SQL is as follows:

SELECT column_name(s)
FROM table1
INNER JOIN table2 ON table1.column_name = table2.column_name;

Let's break down the syntax:

• SELECT: specifies the columns you want to retrieve from the combined tables.
• FROM: indicates the first table you want to join.
• INNER JOIN: specifies the type of join you want to perform.
• table2: represents the second table you want to join.
• ON: specifies the condition for joining the tables. It usually involves a related column between the tables.
• column_name: represents the related column that exists in both tables.

Example

Let's illustrate the concept of INNER JOIN with a practical example. Suppose we have two tables: "Customers" and "Orders". The "Customers" table contains information about customers, while the "Orders" table contains information about their orders. In order to get the data from both tables where there is a match, we can use the INNER JOIN.

Here's an example of how the tables might be structured:

To retrieve the customer's first name, last name, email, order ID, and order date, we could use the following SQL query:

SELECT Customers.FirstName, Customers.LastName, Customers.Email, Orders.OrderID, Orders.OrderDate
FROM Customers
INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID;

In this example, we are selecting the specified columns from the "Customers" and "Orders" table. The INNER JOIN is performed on the "CustomerID" column, which exists in both tables.

The result of this query would be a combined table that includes the requested columns where there is a match between the "CustomerID" column in the "Customers" table and the "CustomerID" column in the "Orders" table.

Why use INNER JOIN?

The INNER JOIN is commonly used in SQL to retrieve data from multiple tables based on their relationships. It allows us to combine information from different tables to create meaningful results. By using INNER JOIN, we can filter and sort data based on the related columns, enabling us to perform complex queries.

By understanding how to use INNER JOIN, you can query your database more effectively and retrieve the data you need by leveraging the relationships between tables. This can be particularly useful when working with large datasets that are spread across multiple tables.

Conclusion

In this article, we explained the concept of INNER JOIN in SQL and how it can be used to combine rows from multiple tables based on a related column. We provided a detailed syntax explanation and presented an example to illustrate its usage. The INNER JOIN is a powerful tool that allows you to retrieve data from different tables based on their relationships, enabling you to perform complex queries and get meaningful results.