Multi-table related query skills in PHP

Multi-table related query skills in PHP

Related query is an important part of database query, especially when you need to display data in multiple related database tables. In PHP applications, multi-table related queries are often used when using databases such as MySQL. The meaning of multi-table association is to compare data in one table with data in another or multiple tables, and connect those rows that meet the requirements in the result.

When performing multi-table correlation queries, you need to consider the relationship between tables and use appropriate correlation methods. The following introduces several techniques for multi-table related queries and how to use them in PHP.

1. Inner join

Inner join is the most commonly used multi-table association method. It matches records in two or more tables and returns only matching ones. OK. In MySQL, the syntax of inner join is as follows:

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

For example, if you need to display a list of customer orders and need to view customer details, you can use the following SQL query:

SELECT customers.CustomerID, customers.CustomerName, orders.OrderDate
FROM customers
INNER JOIN orders
ON customers.CustomerID = orders.CustomerID;

In PHP, this query statement can be executed using the query() method of the mysqli function.

2. Left join

Left join is a commonly used multi-table association method. It returns all rows of the left table and rows related to the left table in the right table. If there is no matching row in the left table, the column in the right table will be NULL. In MySQL, the syntax of a left join is as follows:

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

For example, if you need to display those customers who have no orders in the customer list, and you need to use the following SQL query:

SELECT customers.CustomerName, orders.OrderID
FROM customers
LEFT JOIN orders
ON customers.CustomerID = orders.CustomerID
ORDER BY customers.CustomerName;

In PHP, this query statement can be executed using the query() method of the mysqli function.

3. Right join

Right join is the reverse operation of left join, which returns all rows of the right table and rows in the left table related to the right table. If there is no matching row in the right table, the column in the left table will be NULL. In MySQL, the syntax of a right join is as follows:

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

For example, if you need to display a list of orders and the customer name of the order, and you need to include those orders that have no customers, you can use the following SQL query:

SELECT orders.OrderID, customers.CustomerName
FROM orders
RIGHT JOIN customers
ON orders.CustomerID = customers.CustomerID
ORDER BY orders.OrderID;

In PHP, this query statement can be executed using the query() method of the mysqli function .

4. Full join

Full join matches all rows in the left table and right table and returns all results. If there are no matching rows in the two tables, NULL padding is used. In MySQL, the syntax of a full join is as follows:

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

MySQL does not provide FULL OUTER JOIN, but you can combine left joins and right joins to simulate FULL OUTER JOIN operations.

In PHP, full connection can be achieved using the query() method of the mysqli function.

5. Self-join

Self-join connects two different fields in the same table and creates an alias table to link. The key is to determine the different outputs when using the output Way. In MySQL, the syntax of self-join is as follows:

SELECT table1.column1, table2.column2...
FROM table1, table2
WHERE table1.column = table2.column;

For example, if you want to find employees who have a higher salary than all others, you can use the following SQL query:

SELECT a.employeeName
FROM employee a, employee b
WHERE a.employeeSalary > ; b.employeeSalary
AND b.employeeName = 'Jane';

In PHP, this query statement can be executed using the query() method of the mysqli function.

Summary

Multiple table association queries are very common in PHP and MySQL applications. This article provides five multi-table association techniques, including inner join, left join, right join, Fully connected and self-connected. Use these techniques to better query and display data in multiple related database tables.

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