为什么在执行 Prepared Statement 时会出现 java.sql.SQLSyntaxErrorException？

Question

我的数据库类文件：

package Model;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Arrays;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

public class Database {

private LinkedList<Person> people;

public Database() {
    people = new LinkedList<Person>();
}

private Connection con;

public void connect() throws Exception{

    if(con != null) return;

    try {
        Class.forName("com.mysql.cj.jdbc.Driver");
    } catch (ClassNotFoundException e) {
        throw new Exception("Driver not found");
    }

    String Url = "jdbc:mysql://localhost:3306/jdbc";
    Connection con;
    con = DriverManager.getConnection(Url, "root", "089abc2019");

    System.out.println("Connected: " + con);
}

public void disconnect() {
    if(con != null) {
        try {
            con.close();
        } catch (SQLException e) {
            System.out.println("Can't close connection");
        }
    }
}

public void save() throws SQLException {

    String checkSql = "Select count * from jdbc where id=?";

    String Url = "jdbc:mysql://localhost:3306/jdbc";
    Connection con = DriverManager.getConnection(Url, "root", "089abc2019");

    if (con==null) {
            fail("Connection couldn't be established");
    }

    PreparedStatement checkStmt = con.prepareStatement(checkSql);

    for (Person person: people) {
            int id = person.getId();

            checkStmt.setInt(1, id);

            ResultSet checkResult = checkStmt.executeQuery();
            checkResult.next();

            int count = checkResult.getInt(1);

            System.out.println("Count for person wiht ID: " + id + "is " + count);
        }

        checkStmt.close();
} 

private void fail(String string) {
    System.out.println(string);
}

public void addPerson(Person person) {
    people.add(person);
}

public void removePerson(int index) {
    people.remove();
}

public List<Person> getPeople(){
    return Collections.unmodifiableList(people);
}

public void saveToFile(File file) throws IOException{
    FileOutputStream fos = new FileOutputStream(file);
    ObjectOutputStream oos = new ObjectOutputStream(fos);

    Person[] persons = people.toArray(new Person[people.size()]);

    oos.writeObject(persons);

    oos.close();
}

public void loadFromFile(File file) throws IOException{
    FileInputStream fis = new FileInputStream(file);
    ObjectInputStream ois = new ObjectInputStream(fis);

    try {
        Person[] person = (Person[]) ois.readObject();

        people.clear();

        people.addAll(Arrays.asList(person));

    } catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    ois.close();
}

}

我的 testDatabsae 文件：

import java.sql.SQLException;

import Model.AgeCategory;
import Model.Database;
import Model.EmploymentCategory;
import Model.Gender;
import Model.Person;

public class TestDatabase {

public static void main(String[] args) {

    System.out.println("Running database test");

    Database db = new Database(); 
    try {
        db.connect();
    } catch (Exception e) {
        e.printStackTrace();
    }

    db.addPerson(new Person("Joe", "Builder", AgeCategory.adult, EmploymentCategory.emloyed, "654321", true, Gender.Male));
    db.addPerson(new Person("Sue", "Athlete", AgeCategory.adult, EmploymentCategory.self_Employed, "652321", false, Gender.Female));

    try {
        db.save();
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    db.disconnect();
}

}

控制台中的结果：

Running database test
Connected: com.mysql.cj.jdbc.ConnectionImpl@316bcf94

java.sql.SQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'from jdbc where id=0' at line 1

at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:120)
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122)
at com.mysql.cj.jdbc.ClientPreparedStatement.executeInternal(ClientPreparedStatement.java:953)
at com.mysql.cj.jdbc.ClientPreparedStatement.executeQuery(ClientPreparedStatement.java:1003)
at Model.Database.save(Database.java:74)
at TestDatabase.main(TestDatabase.java:26)

Answer 1

您的 sql 语法不正确。 COUNT是一个组函数。 这个

Select count * from jdbc where id=?

应该

Select count(*) from jdbc where id=?