How to write query statements in php mysql multiple tables

When using PHP and MySQL for data operations, using multi-table query statements can help us better query and operate the data in each data table, thereby expanding the application functions and application scenarios. In this article, we will give a detailed explanation and practical operation of php mysql multi-table query statements.

1. Overview of multi-table queries

In MySQL operations, we often use multiple data tables. Multi-table query refers to the use of multiple data tables when querying data. This is one of the most commonly used methods of SQL syntax, and it is also one of the most basic and important query methods. In multi-table queries, joins are needed to connect multiple data tables to achieve the purpose of data query and operation. There are two ways of connection: internal connection and external connection. Different connection methods are used according to different situations.

2. Multi-table query connection method

1. Inner join query

Inner join query refers to querying by correlating the corresponding data in two tables.

SELECT table1.field1, table2.field2…
FROM table1 INNER JOIN table2
ON table1.field1=table2.field1;

The INNER JOIN here specifies the content to be performed Join query. The ON keyword specifies the two fields to be connected when performing a join query.

2. Left join query

Left join query refers to all records in the left table, and only the corresponding records in the right table are queried.

SELECT table1.field1, table2.field2…
FROM table1 LEFT JOIN table2
ON table1.field1=table2.field1;

LEFT JOIN here represents a left join query. All records in the left table will be retained, and records in the right table that meet the conditions will be queried together. If there is no matching record in the right table, it will be displayed as null.

3. Right join query

Right join query refers to all records in the right table, and only the corresponding records in the left table are queried.

SELECT table1.field1, table2.field2…
FROM table1 RIGHT JOIN table2
ON table1.field1=table2.field1;

The RIGHT JOIN here represents the right join query. All records in the right table will be retained, and records that meet the conditions in the left table will be queried together. Similarly, if there is no matching record in the left table, it will be displayed as null.

4. Full join query

Full join query means that both the left table and the right table must be queried.

SELECT table1.field1, table2.field2…
FROM table1 FULL OUTER JOIN table2
ON table1.field1=table2.field1;

The above FULL OUTER JOIN represents a full join Inquire. Records from both the left table and the right table will be queried.

3. Multi-table query example

Now let’s look at a practical example. Suppose we have two tables, one is the orders table, and the other is the customer table, we can query the values ​​of the fields with the same name by using inner joins. Both sides of the query contain the customer name, and an order record for a customized product is returned.

SELECT orders.orderid, customers.customername
FROM orders
INNER JOIN customers
ON orders.customerid=customers.customerid;

Here we use INNER JOIN specification To perform an inner join query, the ON keyword specifies the fields to connect the two tables.

So, if we want to query all orders of a certain customer, what should we do?

SELECT orders.orderid, orders.orderdate, customers.customername
FROM orders
LEFT JOIN customers
ON orders.customerid=customers.customerid
WHERE customers.customername='John ';

Here we use LEFT JOIN to query all order records, and only display the record with the customer name John. During the join, the orders table is retained, but for the customers table, only records that meet the conditions will be queried.

4. Multi-table query skills

1. Use aliases

Using aliases can improve query speed, reduce query time, and also facilitate our memory and query.

SELECT o.orderid, c.customername
FROM orders AS o
INNER JOIN customers AS c
ON o.customerid=c.customerid;

Here we use AS keyword is used to specify aliases to improve query efficiency and readability.

2. Follow the best query practices

When performing multi-table queries, we need to follow some basic practices, such as reducing query results first, querying only the required data, and avoiding using select * Wait for unnecessary queries; avoid using subqueries, and try to reduce IO consumption during multi-table queries, etc.

3. Optimize query performance

When performing specific queries, we can optimize query performance by optimizing query statements, selecting the most appropriate index, etc.

Summary

In this article we introduced the basic syntax, operation methods and practical applications of php mysql multi-table query statements. When performing multi-table queries, we need to master different join methods and follow best query practices to optimize query performance and improve the accuracy and efficiency of data queries and operations.

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