Question

我正在尝试解决课程的问题，我之前在以下链接中看到了问题：SQL - How to find the person with the highest grade

我看到了提供的答案，但注意到它不能用来帮助我，因为它使用了我不允许使用的连接和顺序，我不能评论这个问题，因为我是新的，所以我再次发布这个问题。

使用以下3个表格查找成绩最高的人员的姓名

People (id, name, age, address)
---------------------------------------------------
p1  | Tom Martin| 24 | 11, Wall Street, New York, NY
p2  | D.J. Tanner  | 33 | 1882, Gerard Street, San Francisco, CA
p3  | Lisa Simpson| 40 | 742, Evergreen Terrace, Springfield, IL
---------------------------------------------------

Courses (cid, name, department)
---------------------------------------------------------
c1 | Systematic Torture | MATH
c2 | Pretty Painful     | CS
c3 | Not so Bad         | MATH
c4 | Big Data           | CS
---------------------------------------------------------

Grades (pid, cid, grade)
---------------------------------------------------
p1  |  c1       | 3.5 
p2  |  c3       | 2.5
p3  |  c2       | 4.0
p3  |  c4       | 3.85
p2  |  c4       | 3.5
p1  |  c4       | 2.5
---------------------------------------------------

我当前的查询：

SELECT DISTINCT People.name
FROM Grades AS g1, Grades AS g2, people AS p1
WHERE g1.cid = 'c4'
AND g2.cid = 'c4'
AND p1.id =- g1.pid
AND g1.grade > g2.grade

我的结果：

Lisa Simpson
Tom Martin

我只需要Lisa Simpson并坚持寻找解决方法。

以下内容无法使用：

COUNT
MAX
MIN
SUM
GROUP BY
HAVING
INNER JOIN
LEFT JOIN
RIGHT JOIN
Anything JOIN
EXISTS
NOT EXISTS
ORDER BY
TOP

只能使用：

SELECT
FROM..AS
WHERE
UNION
INTERSECT
EXCEPT
CREATE VIEW ... AS ...
Arithmetic operators like < > <= ==
AND
OR

基本上他不会让我们使用我们在课堂上没有涉及的任何内容

Answer 1

您将依赖大于所有运算符来执行此操作：

class Xml
    {
        //Function for Read Xml file and store into Sql table
        public void ReadXmlFileAndInsertIntoTable(string client, string xmlpath, string Connectionstring)
        {
            try
            {
                string connetionString = null;
                SqlConnection connection;
                SqlCommand command;
                SqlDataAdapter adpter = new SqlDataAdapter();
                DataSet ds = new DataSet();
                // XmlReader xmlFile;
                string sql = null;

                int product_ID = 0;
                string Product_Name = null;
                double product_Price = 0;

                connetionString = Connectionstring;
                connection = new SqlConnection(connetionString);

                //xmlFile = XmlReader.Create(xmlpath, new XmlReaderSettings());
                //ds.ReadXml(xmlFile);


                string xmlFile = xmlpath;
                XmlDocument xmlDoc = new XmlDocument();
                if (File.Exists(xmlFile))
                {
                    xmlDoc.Load(xmlFile);
                    Console.ForegroundColor = ConsoleColor.Green;
                    Console.WriteLine("\n\t Xml File Read Successfully");

                    XmlNode node = xmlDoc.DocumentElement.FirstChild;
                    XmlNodeList nodelist = node.ChildNodes;

                    //for (int j = 0; j < nodelist.Count; j++)
                    //    Console.WriteLine("{0}", nodelist[j].InnerText);

                    ds.ReadXml(xmlFile);
                }
                else
                {
                    Console.WriteLine("The file {0} could not be located", xmlFile);
                }

                connection.Open();
                int i = 0;
                for (i = 0; i <= ds.Tables[0].Rows.Count - 1; i++)
                {
                    product_ID = Convert.ToInt32(ds.Tables[0].Rows[i].ItemArray[0]);
                    Product_Name = ds.Tables[0].Rows[i].ItemArray[1].ToString();
                    product_Price = Convert.ToDouble(ds.Tables[0].Rows[i].ItemArray[2]);
                    sql = "insert into Product values(" + product_ID + ",'" + Product_Name + "'," + product_Price + ")";
                    command = new SqlCommand(sql, connection);
                    adpter.InsertCommand = command;
                    adpter.InsertCommand.ExecuteNonQuery();
                }
                connection.Close();
                Console.ForegroundColor = ConsoleColor.Green;
                Console.WriteLine("\n\t Data Inserted Successfully...!!!");

            }
            catch(Exception ex)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine(ex.Message);
            }


        }
    }


    //Main Function
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("\n\t=====================================================================");
            Console.WriteLine("\t\t My Application");
            Console.WriteLine("\n\t=====================================================================");

            Console.WriteLine("\t 1. Try Simple XML Data Copy and store in Table");
            Console.WriteLine("\t 2. Exit Tool");

            Console.Write("\n\t Please Enter Your Response :- ");
            int n = Convert.ToInt32(Console.ReadLine());

            switch (n)
            {
                case 1:

                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("\n\t\t Enter Client Name :- ");
                    string client = Console.ReadLine();

                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("\n\t\t Enter XML Path :- ");
                    var xmlpath = Console.ReadLine();

                    Console.ForegroundColor = ConsoleColor.White;
                    Console.Write("\n\t\t Enter Connctionstring :- ");
                    var Connectionstring = Console.ReadLine();

                    Xml xml = new Xml();
                    xml.ReadXmlFileAndInsertIntoTable(client, xmlpath, Connectionstring);
                    break;


                default:
                    Console.ForegroundColor = ConsoleColor.Red;
                    Console.WriteLine("\n\t\t Please enter valid response...!!!");
                    break;
            }
            Console.ReadLine();

        }


    }


}

这是在没有连接，最大和顺序的情况下进行的。

修改：关于使用限制1的说明

限制的使用是因为我正在进行比较select name from people where id = -- this subquery returns the id of the person with the maximum grade (select g.pid from grade g where -- this is an alternative to implement the max function -- It means: where grade is greater then every grade that it is not itself. g.grade > all (select g2.grade from grade g2 where g2.grade <> g.grade) limit 1 )，而某些内容需要与id相同的类型。例如，我无法将int类型的事物与整数列表进行比较。如果我没有使用限制，子查询将返回一个列表，我们有两个最大数字。

Answer 2

SELECT p.name 
  FROM grades a 
  JOIN 
     ( SELECT MAX(grade) grade 
        FROM grades 
     ) b 
    ON b.grade = a.grade 
  JOIN people p 
    ON p.id = a.pid;

MySQL - 如何在没有max，min，join，union等的情况下找到成绩最高的人

2 个答案: