SQLite 查询不在 Java 中运行

Question

我使用 DB Browser for SQLite 创建了一个数据库。 现在我正在尝试运行查询以从数据库中检索数据，但我遇到了一个问题。 当我在 DB Browser 中运行查询时，查询运行顺利。 当我尝试在 Java 中运行相同的查询时，我收到一条错误消息。 我对查询字符串进行了三次检查。 我想确保我没有错误地使用SQLite关键字。 为什么在运行此查询 Java 时出现错误？

SELECT
    subject.id, main.name, main.link, subject.name, main_subject.weight
FROM 
    main_subject
LEFT JOIN 
    main ON main_subject.mainId = main.id
LEFT JOIN 
    subject ON main_subject.subjectId = subject.id

查询在数据库浏览器中正确运行。

MCVE：

主要的

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.io.File;
import java.io.IOException;
import java.lang.reflect.Type;
import java.nio.file.Files;
import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * @author sedrick
 */
public class Main
{

    public static void main(String[] args)
    {
        try (DatabaseHandler databaseHandler = new DatabaseHandler()) {
            List<BySubjectItemTwo> bySubjectItemTwos = databaseHandler.getBySubjectItems();
            bySubjectItemTwos.forEach(System.out::println);
            //databaseHandler.getByTitleItems().forEach(System.out::println);
        }
        catch (Exception ex) {
            Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

数据库处理器

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author sedrick
 */
public class DatabaseHandler implements AutoCloseable
{

    String dbString = "jdbc:sqlite:qDatabase.sqlite3";
    private Connection conn;

    public DatabaseHandler()
    {
        try {
            conn = DriverManager.getConnection(dbString);
            System.out.println("Connected to qDatabase!");
        }
        catch (SQLException ex) {
            Logger.getLogger(DatabaseHandler.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    @Override
    public void close() throws Exception
    {
        conn.close();
    }

    public List<BySubjectItemTwo> getBySubjectItems()
    {
        List<BySubjectItemTwo> returnList = new ArrayList();

        String sqlString = "SELECT subject.id, main.name, main.link, subject.name, main_subject.weight FROM main_subject LEFT JOIN main ON main_subject.mainId = main.id LEFT JOIN subject ON main_subject.subjectId = subject.id";
        try (Statement stmt = conn.createStatement(); ResultSet rs = stmt.executeQuery(sqlString)) {
            while (rs.next()) {
                System.out.println(rs.getInt("subject.id") + " - " + rs.getString("main.name") + " - " + rs.getString("main.link") + " - " + rs.getString("subject.name") + " - " + rs.getInt("main_subject.weight"));
                //returnList.add(new BySubjectItemTwo(rs.getInt("subject.id"), rs.getString("main.name"), rs.getString("main.link"), rs.getString("subject.name"), rs.getInt("main_subject.weight")));
            }
        }
        catch (SQLException e) {
            System.out.println(e.getMessage());
        }

        return returnList;
    }  
}

按主题项目二

/**
 *
 * @author sedrick
 */
public class BySubjectItemTwo
{

    private int subjectId;
    private String mainName;
    private String mainLink;
    private String subjectName;
    private int mainSubjectWeight;

    public BySubjectItemTwo(int subjectId, String mainName, String mainLink, String subjectName, int mainSubjectWeight)
    {
        this.subjectId = subjectId;
        this.mainName = mainName;
        this.mainLink = mainLink;
        this.subjectName = subjectName;
        this.mainSubjectWeight = mainSubjectWeight;
    }

    public int getSubjectWeight()
    {
        return mainSubjectWeight;
    }

    public void setSubjectWeight(int mainSubjectWeight)
    {
        this.mainSubjectWeight = mainSubjectWeight;
    }

    public int getSubjectId()
    {
        return subjectId;
    }

    public void setSubjectId(int subjectId)
    {
        this.subjectId = subjectId;
    }

    public String getMainName()
    {
        return mainName;
    }

    public void setMainName(String mainName)
    {
        this.mainName = mainName;
    }

    public String getMainLink()
    {
        return mainLink;
    }

    public void setMainLink(String mainLink)
    {
        this.mainLink = mainLink;
    }

    public String getSubjectName()
    {
        return subjectName;
    }

    public void setSubjectName(String subjectName)
    {
        this.subjectName = subjectName;
    }

    @Override
    public String toString()
    {
        StringBuilder sb = new StringBuilder();
        sb.append("BySubjectItemTwo{subjectId=").append(subjectId);
        sb.append(", mainName=").append(mainName);
        sb.append(", mainLink=").append(mainLink);
        sb.append(", subjectName=").append(subjectName);
        sb.append(", subjectWeight=").append(mainSubjectWeight);
        sb.append('}');
        return sb.toString();
    }
}

错误

----------< sed.work:CreateDatabaseByTitleAndSubjectDatabase >----------
Building CreateDatabaseByTitleAndSubjectDatabase 1.0-SNAPSHOT
--------------------------------[ jar ]---------------------------------

--- exec-maven-plugin:1.5.0:exec (default-cli) @ CreateDatabaseByTitleAndSubjectDatabase ---
Connected to qDatabase!
[SQLITE_ERROR] SQL error or missing database (ambiguous column name: subject.id)
------------------------------------------------------------------------
BUILD SUCCESS
------------------------------------------------------------------------
Total time:  1.484 s
Finished at: 2020-10-16T12:42:35-05:00
------------------------------------------------------------------------

Netbean 的信息

Product Version: Apache NetBeans IDE 12.1
Java: 15; Java HotSpot(TM) 64-Bit Server VM 15+36-1562
Runtime: Java(TM) SE Runtime Environment 15+36-1562
System: Windows 10 version 10.0 running on amd64; Cp1252; en_US (nb)
Maven Project

Answer 1

这个：

FROM main_subject, main, subject

是您查询的问题。

从您的陈述中删除, main, subject 。
表main和subject在LEFT连接后正确连接。
当您也在FROM子句中使用它们时，这会创建额外的引用和到相同表的CROSS连接。

此外，尽管 SQLite 允许查询返回超过 1 个具有相同名称的列，但最好为列设置别名，以便在结果集中所有列名称都不同。
所以给别名main.name和subject.name ，是这样的：

SELECT subject.id, main.name AS main_name, ..., subject.name AS subject_name, ...

当您检索列值时，不要使用表前缀：

System.out.println(rs.getInt("id") + " - " + rs.getString("main_name") + " - " + rs.getString("link") + " - " + rs.getString("subject_name") + " - " + rs.getInt("weight"));