Detailed introduction to the use of SQL query statements in PHP

PHP is a popular server-side programming language, and MySQL is a widely used relational database management system. Their combination can provide powerful query and data processing capabilities for Web applications. In the PHP development process, SQL query statements are one of the knowledge that must be mastered. This article will introduce in detail how to use SQL query statements in PHP.

1. Basic syntax of query statements

Using SQL query statements in PHP requires the use of mysqli extension or PDO extension. Among them, the mysqli extension is more commonly used. When using the mysqli extension, you need to first establish a connection with the MySQL database, and then execute the SQL query statement through the query() method of the connection object. The basic syntax of the SQL query statement is as follows:

SELECT column1, column2, ... FROM table_name WHERE condition1 AND condition2 ... ORDER BY column_name ASC/DESC

Among them, SELECT is used to specify the field name to be queried; FROM is used to specify the table name to be queried; WHERE is used to specify the conditions of the query; ORDER BY is used to specify How the query results are sorted.

2. Specific use of query statements

1. Query all data

To query all data in the table, you can use The following SQL statement:

SELECT * FROM table_name;

Among them, * represents all fields and table_name represents the table name.

2. Query the specified data

To query the data of the specified field, you can use the following SQL statement:

SELECT column1, column2, ... FROM table_name;

Among them, column1, column2, etc. represent the query Field names, separate multiple field names with commas.

3. Conditional query

To query data that meets the conditions, you can use the WHERE clause. The WHERE clause can be used to limit the scope of data to be queried. The sample code is as follows:

SELECT column1, column2, ... FROM table_name WHERE condition;

Among them, condition represents the query condition. Common query conditions are:

• is equal to: =
• is not equal to: <>
• is greater than: >
• is greater than or equal to: >=
• Less than: <
• Less than or equal to: <=

Multiple conditions can be connected using AND or OR. The sample code is as follows:

SELECT column1, column2, ... FROM table_name WHERE condition1 AND condition2;

4. Sort query

To sort the query results according to the specified field, you can use the ORDER BY clause. ASC means ascending order, DESC means descending order. The sample code is as follows:

SELECT column1, column2, ... FROM table_name ORDER BY column_name ASC/DESC;

Among them, column_name represents the name of the field to be sorted.

5. Limitations of query results

To query a specified amount of data, you can use the LIMIT clause. The sample code is as follows:

SELECT column1, column2, ... FROM table_name LIMIT start_num, record_num;

Among them, start_num represents the starting position of the query, and record_num represents the number of records to be queried.

3. Summary

This article introduces the basic syntax and specific usage of SQL query statements in PHP. By understanding and mastering this knowledge, database operations can be performed more easily and provide stronger support for the development and maintenance of web applications.

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