MySQL - 註釋

Table of content

MySQL 註釋
MySQL 單行註釋
MySQL 多行註釋
註釋放置位置
使用客戶端程式進行註釋

MySQL 註釋

MySQL 註釋是在程式碼段中新增的文字解釋，用於提供有關程式碼的其他資訊。註釋不作為程式碼的一部分執行。它作為開發人員等讀者的註釋，以理解程式碼的目的、功能或任何其他相關細節。

MySQL 中有兩種型別的註釋：單行註釋和多行註釋

MySQL 單行註釋

單行註釋用於在單行上進行簡要解釋。要在 MySQL 中建立單行註釋，請使用兩個連字元 (--) 後跟註釋文字。

示例

在以下查詢中，我們使用單行註釋來編寫文字。

SELECT * FROM customers; -- This is a comment

MySQL 多行註釋

MySQL 中的多行註釋用於較長的解釋或註釋掉多行程式碼。這些註釋以/*開頭，以*/結尾。它們之間的所有內容都被視為註釋。

示例

以下示例使用多行註釋作為查詢的解釋 -

/*
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;
*/

註釋放置位置

您幾乎可以在 SQL 程式碼中的任何位置放置註釋。常見位置包括 -

SQL 語句之前或之後。
在 SQL 語句中解釋其特定部分。
在指令碼或儲存過程的開頭描述其目的。

-- 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 ;

使用客戶端程式進行註釋

我們還可以使用客戶端程式註釋任何值。

語法

PHP NodeJS Java Python

要透過 PHP 程式註釋任何值或查詢，我們需要使用mysqli函式query()執行以下注釋方法，如下所示 -

single line comment
--
multiline comment
/**/

(using Query)

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

要透過 JavaScript 程式註釋任何值或查詢，我們需要使用mysql2庫的query()函式執行以下注釋方法，如下所示 -

single line comment
--
multiline comment
/**/

(using Query)

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

要透過 Java 程式註釋任何值或查詢，我們需要使用JDBC函式executeQuery()執行以下注釋方法，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);

要透過 Python 程式註釋任何值或查詢，我們需要使用MySQL Connector/Python的execute()函式執行以下注釋方法，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

以下是程式 -

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();

輸出

獲得的輸出如下所示 -

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);
    });
});

輸出

獲得的輸出如下所示 -

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://: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();
    }
  }
}

輸出

獲得的輸出如下所示 -

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()

輸出

獲得的輸出如下所示 -

IDs of customers from Mumbai:
4

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