MySQL - Distinct 子句

要通过 Node.js 程序从 MySQL 表中获取不同的记录，我们需要使用 mysql2 库中的 query() 函数执行 SELECT 语句的 DISTINCT 子句，如下所示 -

sql = "SELECT DISTINCT column1, column2, ..., columnN FROM table_name";
con.query(sql);

要通过 Java 程序从 MySQL 表中获取不同的记录，我们需要使用 JDBC 函数 executeUpdate() 执行 SELECT 语句的 DISTINCT 子句，如下所示 -

String sql = "SELECT DISTINCT column1, column2, ..., columnN
FROM table_name WHERE conditions // 可选";
statement.executeQuery(sql);

要通过 Python 程序从 MySQL 表中获取不同的记录，我们需要使用 MySQL Connector/Python 的 execute() 函数执行 SELECT 语句的 DISTINCT 子句，如下所示 -

distinct_clause_query = "SELECT DISTINCT column1, column2, ...FROM table_name"
cursorObj.execute(distinct_clause_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', NULL),(2,'Khilan', 25, 'Delhi', 1500.00),(3,'kaushik', 23, 'Hyderabad', 2000.00),(4,'Chaital', 25, 'Mumbai', NULL),(5,'Hardik', 27, 'Vishakapatnam', 8500.00),(6, 'Komal',22, 'Vishakapatnam', 4500.00),(7, 'Muffy',24, 'Indore', 10000.00);"
  con.query(sql);
 //使用 DISTINCT 运算符
  sql = "SELECT DISTINCT ADDRESS FROM CUSTOMERS;"
  con.query(sql, function(err, result){
    if (err) throw err
    console.log(result)
  });
});             

输出

生成的输出如下 -

Connected!
--------------------------
[
  { ADDRESS: 'Hyderabad' },
  { ADDRESS: 'Delhi' },
  { ADDRESS: 'Mumbai' },
  { ADDRESS: 'Vishakapatnam' },
  { ADDRESS: 'Indore' }
]       

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class DistinctClause {
  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 DISTINCT ADDRESS FROM CUSTOMERS";
            rs = st.executeQuery(sql);
            System.out.println("Distinct records: ");
            while(rs.next()){
              String Address = rs.getString("Address");
              System.out.println("Address: " + Address);
            }
    }catch(Exception e) {
      e.printStackTrace();
    }
  }
}                           

输出

获得的输出如下所示 -

Distinct records: 
Address: Ahmedabad
Address: Delhi
Address: Kota
Address: Mumbai
Address: Bhopal
Address: MP
Address: Indore        

import mysql.connector
#建立连接
connection = mysql.connector.connect(
    host='localhost',
    user='root',
    password='password',
    database='tut'
)
cursorObj = connection.cursor()
distinct_clause_query = """
SELECT DISTINCT ADDRESS
FROM customers
"""
cursorObj.execute(distinct_clause_query)
# 获取所有不同的地址
distinct_addresses = cursorObj.fetchall()
# 打印不同的地址
for address in distinct_addresses:
    print(f"Address: {address[0]}")
cursorObj.close()
connection.close()                                        

输出

以下是上述代码的输出 -

Address: Hyderabad
Address: Kerala
Address: Mumbai
Address: Vishakapatnam
Address: Delhi