MySQL - 注释

Table of content

MySQL 注释
MySQL 单行注释
MySQL 多行注释
注释放置位置
使用客户端程序进行注释

MySQL 注释

MySQL 注释是在代码段中添加的文本解释，用于提供有关代码的其他信息。注释不作为代码的一部分执行。它作为开发人员等读者的注释，以理解代码的目的、功能或任何其他相关细节。

MySQL 中有两种类型的注释：单行注释和多行注释

MySQL 单行注释

单行注释用于在单行上进行简要解释。要在 MySQL 中创建单行注释，请使用两个连字符 (--) 后跟注释文本。

示例

在以下查询中，我们使用单行注释来编写文本。

SELECT * FROM customers; -- This is a comment

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MySQL 多行注释

MySQL 中的多行注释用于较长的解释或注释掉多行代码。这些注释以/*开头，以*/结尾。它们之间的所有内容都被视为注释。

示例

以下示例使用多行注释作为查询的解释 -

/*
This is a multi-line comment.
You can use it to explain complex queries or comment out multiple lines of code.

SELECT *
FROM products
WHERE price > 50;
*/

注释放置位置

您几乎可以在 SQL 代码中的任何位置放置注释。常见位置包括 -

SQL 语句之前或之后。
在 SQL 语句中解释其特定部分。
在脚本或存储过程的开头描述其目的。

-- This is a comment before a query
SELECT * FROM orders;

SELECT /* This is an inline comment */ customer_name
FROM customers;

/* This is a comment block at the beginning of a script */
DELIMITER //
CREATE PROCEDURE CalculateDiscount(IN product_id INT)
BEGIN
    -- Calculate discount logic here
END //
DELIMITER ;

使用客户端程序进行注释

我们还可以使用客户端程序注释任何值。

语法

PHP NodeJS Java Python

要通过 PHP 程序注释任何值或查询，我们需要使用mysqli函数query()执行以下注释方法，如下所示 -

single line comment
--
multiline comment
/**/

(using Query)

$sql = "SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'";
$mysqli->query($sql);

要通过 JavaScript 程序注释任何值或查询，我们需要使用mysql2库的query()函数执行以下注释方法，如下所示 -

single line comment
--
multiline comment
/**/

(using Query)

sql = "SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'";
con.query(sql);

要通过 Java 程序注释任何值或查询，我们需要使用JDBC函数executeQuery()执行以下注释方法，如下所示 -

single line comment
--
multiline comment
/**/

(using Query)

String sql = "SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'";
statement.executeQuery(sql);

要通过 Python 程序注释任何值或查询，我们需要使用MySQL Connector/Python的execute()函数执行以下注释方法，如下所示 -

single line comment
--
multiline comment
/**/

(using Query)

comments_query = "SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'"
cursorObj.execute(comments_query)

示例

以下是程序 -

PHP NodeJS Java Python

$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = 'password';
$db = 'TUTORIALS';
$mysqli = new mysqli($dbhost, $dbuser, $dbpass, $db);
if ($mysqli->connect_errno) {
    printf("Connect failed: %s", $mysqli->connect_error);
    exit();
}
//printf('Connected successfully.');
$sql = "SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'";
if($mysqli->query($sql)){
    printf("Select query executed successfully...!\n");
}
printf("Table records: \n");
if($result = $mysqli->query($sql)){
    while($row = mysqli_fetch_array($result)){
        printf("Id: %d",
        $row['ID']);
        printf("\n");
    }
}
if($mysqli->error){
    printf("Error message: ", $mysqli->error);
}
$mysqli->close();

输出

获得的输出如下所示 -

Select query executed successfully...!
Table records:
Id: 4

var mysql = require('mysql2');
var con = mysql.createConnection({
host:"localhost",
user:"root",
password:"password"
});

 //Connecting to MySQL
 con.connect(function(err) {
 if (err) throw err;
//   console.log("Connected successfully...!");
//   console.log("--------------------------");
 sql = "USE TUTORIALS";
 con.query(sql);
 //create table
 sql = "SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'";
 con.query(sql, function(err, result){
    console.log("Select query executed successfully(where we commented the name and address column)...!");
    console.log("Table records: ")
    if (err) throw err;
    console.log(result);
    });
});

输出

获得的输出如下所示 -

Select query executed successfully(where we commented the name and address column)...!
Table records:
[ { ID: 4 } ]

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class Comments {
  public static void main(String[] args) {
    String url = "jdbc:mysql://localhost:3306/TUTORIALS";
    String user = "root";
    String password = "password";
    ResultSet rs;
    try {
      Class.forName("com.mysql.cj.jdbc.Driver");
            Connection con = DriverManager.getConnection(url, user, password);
            Statement st = con.createStatement();
            //System.out.println("Database connected successfully...!");
            //create table
            String sql = "SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'";
            rs = st.executeQuery(sql);
            System.out.println("Table records: ");
            while(rs.next()) {
              String id = rs.getString("id");
              System.out.println("Id: " + id);
            }
    }catch(Exception e) {
      e.printStackTrace();
    }
  }
}

输出

获得的输出如下所示 -

Table records: 
Id: 4

import mysql.connector
# Establishing the connection
connection = mysql.connector.connect(
    host='localhost',
    user='root',
    password='password',
    database='tut'
)
cursorObj = connection.cursor()
# Query with comments
comments_query = """SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'"""
cursorObj.execute(comments_query)
# Fetching all the rows that meet the criteria
filtered_rows = cursorObj.fetchall()
# Printing the result
print("IDs of customers from Mumbai:")
for row in filtered_rows:
    print(row[0])
cursorObj.close()
connection.close()

输出

获得的输出如下所示 -

IDs of customers from Mumbai:
4

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