How to use JOIN statement to query multiple tables at the same time in PHP

In PHP, we sometimes need to query data in multiple database tables at the same time. In this case, we can use the JOIN operation, which allows us to perform join queries in multiple tables. In this article, we will introduce how to use JOIN operation in PHP to realize simultaneous query of multiple tables.

The JOIN operation is a basic operation in the SQL query language, allowing us to obtain the required data by connecting multiple tables. In PHP, we can use MySQLi or PDO to connect to the MySQL database and perform JOIN operations.

In MySQL, there are several different types of JOIN operations, including INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL OUTER JOIN. Here, we will introduce two of them: INNER JOIN and LEFT JOIN.

INNER JOIN

INNER JOIN is the most commonly used JOIN operation. It returns only those records that have a match in the join criteria. The INNER JOIN operation can be used to easily connect data in multiple tables.

For example, we have an orders table that stores order information, and a customers table that stores customer information. There is a field customer_id in the orders table, which is used to represent the customer ID corresponding to each order. We need to query the details of each order and the name of the corresponding customer.

First, we need to use the SELECT statement to select the required fields, and then use the INNER JOIN operation to associate the two tables. The connection condition is that the customer_id field of the orders table is equal to the id field of the customers table.

SELECT o.*, c.name FROM orders o
INNER JOIN customers c
ON o.customer_id = c.id;

In the INNER JOIN operation, the table after the JOIN keyword is the table to be connected, and the ON keyword is followed by the connection condition.

LEFT JOIN

LEFT JOIN is similar to INNER JOIN, but the difference is that it returns all records from the left table (that is, the table preceding the JOIN keyword), Even though there is no matching record in the right table.

For example, we now need to query the details of each customer and their recent order information. We use the LEFT JOIN operation to connect the customers table and the orders table. The connection condition is that the customer_id field of the orders table is equal to the id field of the customers table.

SELECT c.*, o.order_date FROM customers c
LEFT JOIN orders o
ON c.id = o.customer_id
ORDER BY c.id;

In the above example, we selected all fields of the customers table and the order_date field of the orders table. Because of the LEFT JOIN operation, even if some customers have no order records, they will still appear in the result set. The result set will be sorted by the id field of the customers table.

Summary

Using the JOIN operation, you can easily query data in multiple tables. The INNER JOIN operation returns only records that have a match in the join condition, while the LEFT JOIN operation returns all records in the left table, even if there are no matching records in the right table. In PHP, we can use MySQLi or PDO to connect to the MySQL database and perform JOIN operations. The above is a brief introduction to using JOIN operation to realize simultaneous query of multiple tables in PHP.

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