MySQL - Exists 运算符

要通过 Node.js 程序验证 MySQL 表中是否存在特定记录，我们需要使用 mysql2 库中的 query() 函数执行带有 EXISTS 运算符的 SELECT 语句，如下所示 -

sql= "SELECT column1, column2, ... FROM table_name
WHERE EXISTS (subquery)";
con.query(sql);

要通过 Java 程序验证 MySQL 表中是否存在特定记录，我们需要使用 JDBC 函数 executeUpdate() 执行带有 EXISTS 运算符的 SELECT 语句，如下所示 -

String sql = "SELECT column1, column2, ... FROM table_name
WHERE EXISTS (subquery)";
statement.executeQuery(sql);

要通过 Python 程序验证 MySQL 表中是否存在特定记录，我们需要使用 MySQL Connector/Python 的 execute() 函数执行带有 EXISTS 运算符的 SELECT 语句，如下所示 -

exists_query = "SELECT column1, column2, ... FROM table_name
WHERE EXISTS (subquery)"
cursorObj.execute(exists_query);

var mysql = require('mysql2');
var con = mysql.createConnection({
    host: "localhost",
    user: "root",
    password: "Nr5a0204@123"
});
 //连接到 MySQL
  con.connect(function (err) {
  if (err) throw err;
  console.log("Connected!");
  console.log("--------------------------");
 //创建数据库
  sql = "create database TUTORIALS"
  con.query(sql);
 //选择数据库
  sql = "USE TUTORIALS"
  con.query(sql);
 //创建 CUSTOMERS 表
  sql = "CREATE TABLE CUSTOMERS (ID INT NOT NULL, NAME VARCHAR(20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR(25), SALARY DECIMAL(18, 2), PRIMARY KEY(ID));"
  con.query(sql);
 //插入记录
  sql = "INSERT INTO CUSTOMERS(ID, NAME, AGE, ADDRESS, SALARY) VALUES(1,'Ramesh', 32, 'Hyderabad',4000.00),(2,'Khilan', 25, 'Kerala', 8000.00),(3,'kaushik', 23, 'Hyderabad', 11000.00),(4,'Chaital', 25, 'Mumbai', 1200.00),(5,'Hardik', 27, 'Vishakapatnam', 10000.00),(6, 'Komal',29, 'Vishakapatnam', 7000.00),(7, 'Muffy',24, 'Delhi', 10000.00);"
  con.query(sql);
 //Creating CARS table
  sql = "CREATE TABLE CARS(ID INT NOT NULL,NAME VARCHAR(20) NOT NULL,PRICE INT NOT NULL,PRIMARY KEY (ID));"
  con.query(sql);
  
  //插入记录
  sql = "INSERT INTO CARS VALUES(2, 'Maruti Swift', 450000),(4, 'VOLVO', 2250000),(7, 'Toyota', 2400000);"
  con.query(sql);
 //使用 EXISTS 运算符
  sql = "SELECT * FROM CUSTOMERS WHERE EXISTS (SELECT PRICE FROM CARS WHERE CARS.ID = CUSTOMERS.ID AND PRICE > 2000000);"
  con.query(sql, function(err, result){
    if (err) throw err
    console.log(result)
  });
});    

输出

生成的输出如下 -

Connected!
--------------------------
[
  {
    ID: 4,
    NAME: 'Chaital',
    AGE: 25,
    ADDRESS: 'Mumbai',
    SALARY: '1200.00'
  },
  {
    ID: 7,
    NAME: 'Muffy',
    AGE: 24,
    ADDRESS: 'Delhi',
    SALARY: '10000.00'
  }
]          

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class ExistsOperator {
  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...!");
            String sql = "SELECT * FROM CUSTOMERS WHERE EXISTS (SELECT PRICE FROM CARS WHERE CARS.ID = CUSTOMERS.ID AND PRICE > 2000000)";
            rs = st.executeQuery(sql);
            System.out.println("Table records: ");
            while(rs.next()) {
              String id = rs.getString("Id");
              String name = rs.getString("Name");
              String age = rs.getString("Age");
              String address = rs.getString("Address");
              String salary = rs.getString("Salary");
              System.out.println("Id: " + id + ", Name: " + name + ", Age: " + age + ", Addresss: " + address + ", Salary: " + salary);
            }
    }catch(Exception e) {
      e.printStackTrace();
    }
  }
}                                   

输出

获得的输出如下所示 -

Table records: 
Id: 4, Name: Chaitali, Age: 30, Addresss: Mumbai, Salary: 6500.00
Id: 7, Name: Muffy, Age: 24, Addresss: Indore, Salary: 10000.00             

import mysql.connector
#建立连接
connection = mysql.connector.connect(
    host='localhost',
    user='root',
    password='password',
    database='tut'
)
cursorObj = connection.cursor()
exists_query = f"""
SELECT * FROM CUSTOMERS WHERE EXISTS (SELECT PRICE FROM CARS WHERE 
CARS.ID = CUSTOMERS.ID AND PRICE > 2000000); """
cursorObj.execute(exists_query)
# 获取所有符合条件的行
filtered_rows = cursorObj.fetchall()
for row in filtered_rows:
    print(row)
cursorObj.close()
connection.close()                                

输出

以下是上述代码的输出 -

(4, 'Chaital', 25, 'Mumbai', Decimal('1200.00'))
(7, 'Muffy', 24, 'Delhi', Decimal('10000.00'))