MySQL - 左连接

要通过 JavaScript 程序使用左连接连接两个表，我们需要使用mysql2库的query()函数执行带有 LEFT JOIN 子句的 SQL 查询，如下所示：

sql = "SELECT a.tutorial_id, a.tutorial_author, b.tutorial_count FROM tutorials_tbl a LEFT JOIN tcount_tbl b 
ON a.tutorial_author = b.tutorial_author";
con.query(sql);  

要通过 Java 程序使用左连接连接两个表，我们需要使用JDBC函数executeQuery()执行带有 LEFT JOIN 子句的 SQL 查询，如下所示：

String sql = "SELECT a.tutorial_id, a.tutorial_author, b.tutorial_count FROM tutorials_tbl a LEFT JOIN tcount_tbl b 
ON a.tutorial_author = b.tutorial_author";
st.executeQuery(sql);

要使用 Python 程序通过左连接连接两个表，我们需要使用 **execute()** 函数和 **MySQL Connector/Python** 执行带有 LEFT JOIN 子句的 SQL 查询，如下所示：

left_join_query = "SELECT ID, NAME, AMOUNT, DATE FROM CUSTOMERS LEFT JOIN ORDERS ON CUSTOMERS.ID = ORDERS.CUST_ID"
cursorObj.execute(left_join_query)

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);
  //left join
  sql =
    "SELECT a.tutorial_id, a.tutorial_author, b.tutorial_count FROM tutorials_tbl a LEFT JOIN tcount_tbl b ON a.tutorial_author = b.tutorial_author";
  con.query(sql, function (err, result) {
    if (err) throw err;
    console.log(result);
  });
});

输出

产生的输出如下：

[
   { tutorial_id: 1, tutorial_author: 'John Poul', tutorial_count: 2 },
   { tutorial_id: 2, tutorial_author: 'Abdul S', tutorial_count: null },
   {
     tutorial_id: 101,
     tutorial_author: 'Aman kumar',
     tutorial_count: null
   },
   {
     tutorial_id: 102,
     tutorial_author: 'Sarika Singh',
     tutorial_count: null
   }
]

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;

public class LeftJoin {
   public static void main(String[] args) {
      String url = "jdbc:mysql://127.0.0.1:3306/TUTORIALS";
      String username = "root";
      String password = "password";
      try {
         Class.forName("com.mysql.cj.jdbc.Driver");
         Connection connection = DriverManager.getConnection(url, username, password);
         Statement statement = connection.createStatement();
         System.out.println("Connected successfully...!");

         //MySQL LEFT JOIN...!;
         String sql = "SELECT a.tutorial_id, a.tutorial_author, b.tutorial_count FROM tutorials_tbl a LEFT JOIN tcount_tbl b ON a.tutorial_author = b.tutorial_author";
         ResultSet resultSet = statement.executeQuery(sql);
         System.out.println("Table records after LEFT Join...!");
         while (resultSet.next()){
            System.out.println(resultSet.getString(1)+ " "+ resultSet.getString(2)+" "+resultSet.getString(3));
         }
         connection.close();
      } catch (Exception e) {
         System.out.println(e);
      }
   }
}       

输出

获得的输出如下所示：

Connected successfully...!
Table records after LEFT Join...!
1 John Paul 1
2 Abdul S null
3 Sanjay 1
4 Sasha Lee null
5 Chris Welsh null

import mysql.connector
#establishing the connection
connection = mysql.connector.connect(
    host='localhost',
    user='root',
    password='password',
    database='tut'
)
cursorObj = connection.cursor()
left_join_query = f"""
SELECT ID, NAME, AMOUNT, DATE
FROM CUSTOMERS
LEFT JOIN ORDERS
ON CUSTOMERS.ID = ORDERS.CUST_ID
"""
cursorObj.execute(left_join_query)
# Fetching all the rows that meet the criteria
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：

(1, 'Ramesh', None, None)
(2, 'Khilan', 1560, '2009-11-20 00:00:00')
(3, 'Kaushik', 1500, '2009-10-08 00:00:00')
(3, 'Kaushik', 3000, '2009-10-08 00:00:00')
(4, 'Chaital', 2060, '2008-05-20 00:00:00')
(5, 'Hardik', None, None)
(6, 'Komal', None, None)
(7, 'Muffy', None, None)