How to write related query statements using PHP

PHP is a very popular back-end language that can use associated query statements to query data from multiple tables and join them together. This article will introduce you to the concept of related query statements and how to write related query statements using PHP.

1. What is a related query?

Associative query is a query method that queries data in multiple tables and combines them into a result set. Using related queries, developers can efficiently retrieve records to reference and analyze data between different related tables.

In a related query, multiple tables can be connected through a common value, which can be a primary key, a foreign key, or other related values. Through related queries, the data in each table can be connected, making the query results more flexible and complete.

2. Related query statements in PHP

In PHP, we can use SQL statements to implement related queries. The basic syntax of the SQL statement is as follows:

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

In this statement, column_name(s) is the column you want to retrieve, table1 and table2 are the tables you want to connect, and the connection condition is in the ON clause Specified.

JOIN is the connection type, there are the following types:

1. INNER JOIN: Returns the matching rows between the two tables
2. LEFT JOIN: Returns the rows of the left table All rows and matching rows of the right table
3. RIGHT JOIN: Returns all rows of the right table and matching rows of the left table
4. FULL OUTER JOIN: Returns all rows of the left and right tables

Let's look at a few specific examples:

Example 1: Two table connection query

Suppose we have two tables, one is the "users" table and the other In the "orders" table, we want to find out the number of orders placed by each user. You can use the following SQL query statement:

SELECT users.id, users.name, COUNT(orders.order_id) AS order_count
FROM users
LEFT JOIN orders ON users.id = orders.user_id
GROUP BY users.name
ORDER BY order_count DESC;

This SQL query statement uses LEFT JOIN to connect the "users" table and the "orders" table. Then use the corresponding data of GROUP BY to group, use COUNT to count the number of user orders, and finally sort by the number of orders.

Example 2: Three table connection query

As another example, suppose we have three tables, namely "users", "orders" and "order_details". We want to To query the total order amount of each user in the "users" table. You can use the following SQL statement:

SELECT u.id, u.name, SUM(od.price * od.quantity) AS total_price
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
LEFT JOIN order_details od ON o.order_id = od.order_id
GROUP BY u.name

This SQL query statement uses two LEFT JOINs to connect three tables. Then use GROUP BY to group the corresponding data, use SUM to count the total order amount, and sort by user name.

3. Conclusion

Related query is an effective way to query data in multiple tables, and it is also one of the skills that web developers must master. This article briefly introduces the concept and use of related query statements. In actual development, developers also need to optimize and adjust based on specific scenarios to better meet the needs of the project.

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