MySQL - NULL 值

要通过 JavaScript 程序检查某个列的值是否为空，我们需要使用 mysql2 库的 query() 函数执行"SELECT"语句，如下所示 -

sql = "SELECT * from tcount_tbl WHERE tutorial_count IS NOT NULL";
con.query(sql);

要通过 Java 程序检查某个列的值是否为空，我们需要使用 JDBC 函数 executeQuery() 执行"SELECT"语句，如下所示 -

String sql = "SELECT * FROM tcount_tbl WHERE tutorial_count IS NULL";
statement.executeQuery(sql);

要通过 Python 程序检查某个列的值是否为空，我们需要使用 MySQL Connector/Python 的 execute() 函数执行"SELECT"语句，如下所示:−

null_values_query = "SELECT * FROM tcount_tbl WHERE tutorial_count IS NULL"
cursorObj.execute(null_values_query)

var mysql = require('mysql2');
var con = mysql.createConnection({
host:"localhost",
user:"root",
password:"password"
});
 //连接到 MySQL
 con.connect(function(err) {
 if (err) throw err;
  //console.log("Connected successfully...!");
  //console.log("--------------------------");
 sql = "USE TUTORIALS";
 con.query(sql);
 sql = "SELECT * from tcount_tbl WHERE tutorial_count IS NOT NULL";
 if(con.query(sql)){
    console.log("Select query executed successfully....!");
 }
 else{
    console.log("Error");
 }
 console.log("Table records: ");
 con.query(sql, function(err, result){
 if (err) throw err;
 console.log(result);
 });
});                     

输出

获得的输出如下所示 -

Select query executed successfully....!
Table records: 
[
  { tutorial_author: 'mahran', tutorial_count: 20 },
  { tutorial_author: 'Gill', tutorial_count: 20 }
]

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class NullValues {
   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 tcount_tbl WHERE tutorial_count = NULL";
            rs = st.executeQuery(sql);
            System.out.println("Table records(tutorial_count = null): ");
            while(rs.next()) {
               String tutorial_author = rs.getString("tutorial_author");
               String tutorial_count = rs.getString("tutorial_count");
               System.out.println("Author: " + tutorial_author + ", Tutorial_count: " + tutorial_count);
            }
            String sql1 = "SELECT * FROM tcount_tbl WHERE tutorial_count IS NULL";
            rs = st.executeQuery(sql1);
            System.out.println("Table records(tutorial_count is null): ");
            while(rs.next()) {
               String tutorial_author = rs.getString("tutorial_author");
               String tutorial_count = rs.getString("tutorial_count");
               System.out.println("Author: " + tutorial_author + ", Tutorial_count: " + tutorial_count);
            }
      }catch(Exception e) {
         e.printStackTrace();
      }
   }
}      

输出

获得的输出如下所示 -

This will generate the following output - 
Table records(tutorial_count = null): 
Table records(tutorial_count is null): 
Author: mahnaz, Tutorial_count: null
Author: Jen, Tutorial_count: null

import mysql.connector
#建立连接
connection = mysql.connector.connect(
    host='localhost',
    user='root',
    password='password',
    database='tut'
)
cursorObj = connection.cursor()
null_values_query = f"""
SELECT * FROM tcount_tbl 
WHERE tutorial_count IS NULL
"""
cursorObj.execute(null_values_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

获得的输出如下所示 -

('mahnaz', None)
('Jen', None)