select 查询内容 from where 查询条件

对select 查询内容 from
代码

#include <iostream>
#include <string>
#include <fstream>
#include <set>
#include <sstream>
#include<vector>
using namespace std;


bool isDelimiter(char c) {

    if (c == ' ' || c == ',')
    {  
        return true;
    }
    else {
        return false;
    }
}

vector<string> splitString(const string& str)
{
    vector<string>tokens;
    string token;

    for (char c : str) {  //挨个从str中取，判断是否是分隔符
        if (isDelimiter(c) == 1) {
            if (!token.empty()) {
                tokens.push_back(token);
                token.clear();
            }
        }
        else {
            token += c;
        }
    }
    if (!token.empty()) {
        tokens.push_back(token);
    }

    return tokens;
}


struct Student {
    string studentID;
    string name;
    char gender;
    int score;
    Student(string ID, string n, char g, int s) :studentID(ID), name(n), gender(g), score(s) {}
    bool operator<(const Student& other) const {
        return studentID < other.studentID;
    }
};


set<Student> readData(const string& filename) {
    //按行读取infile文件
    string line;
    ifstream infile(filename);
    set<Student> students;

    if (!infile.is_open()) {  //文件不能打开
        cerr << "infile open err" << endl;
    }

    while (getline(infile, line)) //只要有数据
    {

        istringstream iss(line);
        string studentID, name;
        char gender;
        int score;
        if (iss >> studentID >> name >> gender >> score) {
            students.insert(Student(studentID, name, gender, score)); //读取
        }
    }
    infile.close();
    return students;
}

void printFindColumns(set<Student>& studentSet, vector<string>& findColumns)
{
    for (const auto& student : studentSet) {
        for (const auto& coulumn : findColumns) {
            if (coulumn == "id") {
                cout << student.studentID << " ";
            }
            else if (coulumn=="name") {
                cout <<student.name << " ";
            }
            else if (coulumn == "score") {
                cout << student.score << " ";
            }
            else if (coulumn == "gender") {
                cout << student.gender << " ";
            }
            else {
                cout << "属性不存在" << endl;
            }
        }
        cout << endl;
    }
}

int main()
{
    set<Student>studentSet;
    string filename = "input.txt";
    studentSet = readData(filename);  //读取文件存储
    
    string query;
    cout << "please input SQL:"<<endl;
    getline(cin, query);

    string selectword = "select";
    string fromword = "from";

    int select_pos = query.find(selectword);
    int from_pos = query.find(fromword);


    //扣出select和from 中间的字符串
    string columnNames;
    columnNames = query.substr(select_pos + size(selectword), from_pos - (select_pos + size(selectword)));
    
    vector<string> findColumns = splitString(columnNames);
    for (const auto& token : findColumns) {
        cout << token << "   ";
    }
    cout << endl;

    printFindColumns(studentSet, findColumns);

    

    return 1;
}

现在要在这个基础上后面加一个where查询条件该怎么改一下

#include <iostream>
#include <string>
#include <fstream>
#include <set>
#include <sstream>
#include<vector>
using namespace std;


bool isDelimiter(char c) {

    if (c == ' ' || c == ',')
    {
        return true;
    }
    else {
        return false;
    }
}

vector<string> splitString(const string& str)
{
    vector<string>tokens;
    string token;

    for (char c : str) {  //挨个从str中取，判断是否是分隔符
        if (isDelimiter(c) == 1) {
            if (!token.empty()) {
                tokens.push_back(token);
                token.clear();
            }
        }
        else {
            token += c;
        }
    }
    if (!token.empty()) {
        tokens.push_back(token);
    }

    return tokens;
}


struct Student {
    string studentID;
    string name;
    char gender;
    int score;
    Student(string ID, string n, char g, int s) :studentID(ID), name(n), gender(g), score(s) {}
    bool operator<(const Student& other) const {
        return studentID < other.studentID;
    }
};


set<Student> readData(const string& filename) {
    //按行读取infile文件
    string line;
    ifstream infile(filename);
    set<Student> students;

    if (!infile.is_open()) {  //文件不能打开
        cerr << "infile open err" << endl;
    }

    while (getline(infile, line)) //只要有数据
    {

        istringstream iss(line);
        string studentID, name;
        char gender;
        int score;
        if (iss >> studentID >> name >> gender >> score) {
            students.insert(Student(studentID, name, gender, score)); //读取
        }
    }
    infile.close();
    return students;
}

void printFindColumns(set<Student>& studentSet, vector<string>& findColumns)
{
    for (const auto& student : studentSet) {
        for (const auto& coulumn : findColumns) {
            if (coulumn == "id") {
                cout << student.studentID << " ";
            }
            else if (coulumn == "name") {
                cout << student.name << " ";
            }
            else if (coulumn == "score") {
                cout << student.score << " ";
            }
            else if (coulumn == "gender") {
                cout << student.gender << " ";
            }
            else {
                cout << "属性不存在" << endl;
            }
        }
        cout << endl;
    }
}

void printFindnames(set<Student>& studentSet, vector<string>& findColumns, vector<string>& findnames)
{
    for (const auto& student : studentSet) {
        

        cout << endl;
    }
}

int main()
{
    set<Student>studentSet;
    string filename = "input.txt";
    studentSet = readData(filename);  //读取文件存储

    string query;
    cout << "please input SQL:" << endl;
    getline(cin, query);

    string selectword = "select";
    string fromword = "from";
    string whereword = "where";

    int select_pos = query.find(selectword); //select 首字母在的位置
    int from_pos = query.find(fromword);   //from 首字母在的位置
    int where_pos = query.find(whereword);  //where的位置

    //扣出select和from 中间的字符串
    string columnNames;
    columnNames = query.substr(select_pos + size(selectword), from_pos - (select_pos + size(selectword)));
    //扣出where后面的字符串
    string wherenames;
    wherenames = query.substr(where_pos + size(whereword), size(query) - (where_pos + size(whereword)));
    cout << wherenames << endl;

    //去除掉多余的空格和逗号
    vector<string> findColumns = splitString(columnNames);
    vector<string> findnames = splitString(wherenames);
    for (const auto& token : findColumns) {
        cout << token << "    ";
    }
    for (const auto& token : findnames) {
        cout << token << "    ";
    }
    cout << endl;



    printFindnames(studentSet, findColumns,findnames);



    return 1;
}

应该是在这个函数中改

void printFindnames(set<Student>& studentSet, vector<string>& findColumns, vector<string>& findnames)
{
    for (const auto& student : studentSet) {
        

        cout << endl;
    }
}

• 这有个类似的问题, 你可以参考下: https://ask.csdn.net/questions/7651003
• 这篇博客也不错, 你可以看下select查询不到实际存在的表
• 除此之外, 这篇博客: select查询之分页查询中的 案例 部分也许能够解决你的问题, 你可以仔细阅读以下内容或跳转源博客中阅读:
  

• 有奖金的员工信息，并且工资较高的前10名显示出来
  select * from employees
  where commission_pct is not null
  order by salary desc
  limit 0,10;