java - 如何在 SQLite 中向表添加外键？

Question

我创建了两个表，分别称为 TermTable 和 CourseTable。我希望 CourseTable 有一个外键来引用 TermTable。

这是我创建表的代码:

public void onCreate(SQLiteDatabase db) {
        db.execSQL("create table " + TermDbSchema.TermTable.NAME + "(" +
                "_id integer primary key autoincrement, " +
                TermDbSchema.TermTable.Cols.UUID + ", " +
                TermDbSchema.TermTable.Cols.TITLE + ", " +
                TermDbSchema.TermTable.Cols.START_DATE + ", " +
                TermDbSchema.TermTable.Cols.END_DATE +
                ")"
        );
//I want to give CourseTable a foreign key to reference TermTable^^
        db.execSQL("create table " + TermDbSchema.CourseTable.NAME + "(" +
                "_id integer primary key autoincrement, " +
                TermDbSchema.CourseTable.Cols.UUID + ", " +
                TermDbSchema.CourseTable.Cols.TITLE + ", " +
                TermDbSchema.CourseTable.Cols.START_DATE + ", " +
                TermDbSchema.CourseTable.Cols.END_DATE + ", " +
                TermDbSchema.CourseTable.Cols.COURSE_STATUS + ", " +
                TermDbSchema.CourseTable.Cols.OPTIONAL_NOTE + ", " +
                TermDbSchema.CourseTable.Cols.MENTOR_NAME + ", " +
                TermDbSchema.CourseTable.Cols.MENTOR_PHONE + ", " +
                TermDbSchema.CourseTable.Cols.MENTOR_EMAIL +

                ")"
        );
}

这是代码架构代码:

public class TermDbSchema {

public static final class TermTable {
        public static final String NAME = "terms";
        public static final class Cols {
            public static final String UUID = "uuid";
            public static final String TITLE = "title";
            public static final String START_DATE = "startdate";
            public static final String END_DATE = "enddate";
        }
    }
    public static final class CourseTable {
        public static final String NAME = "courses";
        public static final class Cols {
            public static final String UUID = "uuid";
            public static final String TITLE = "title";
            public static final String START_DATE = "startdate";
            public static final String END_DATE = "enddate";
            public static final String COURSE_STATUS = "coursestatus";
            public static final String OPTIONAL_NOTE = "optionalnote";
            public static final String MENTOR_NAME = "mentorname";
            public static final String MENTOR_PHONE = "mentorphone";
            public static final String MENTOR_EMAIL = "mentoremail";        
        }
}

如何向 CourseTable 添加外键，以便它可以引用 TermTable？

Answer 1

更改 CourseTable 以添加另一列作为引用(链接、关系、关联、映射都是其他术语) :-

public static final class CourseTable {
    public static final String NAME = "courses";
    public static final class Cols {
        public static final String UUID = "uuid";
        public static final String TITLE = "title";
        public static final String START_DATE = "startdate";
        public static final String END_DATE = "enddate";
        public static final String COURSE_STATUS = "coursestatus";
        public static final String OPTIONAL_NOTE = "optionalnote";
        public static final String MENTOR_NAME = "mentorname";
        public static final String MENTOR_PHONE = "mentorphone";
        public static final String MENTOR_EMAIL = "mentoremail"; 
        public static final String TERM_LINK = "termlink"    //<<<<<<<<<< ADDED   
    }

• 列名当然可以是您想要的

更改为类(class)表创建 SQL 以添加外键约束。

         db.execSQL("create table " + TermDbSchema.CourseTable.NAME + "(" +
            "_id integer primary key autoincrement, " +
            TermDbSchema.CourseTable.Cols.UUID + ", " +
            TermDbSchema.CourseTable.Cols.TITLE + ", " +
            TermDbSchema.CourseTable.Cols.START_DATE + ", " +
            TermDbSchema.CourseTable.Cols.END_DATE + ", " +
            TermDbSchema.CourseTable.Cols.COURSE_STATUS + ", " +
            TermDbSchema.CourseTable.Cols.OPTIONAL_NOTE + ", " +
            TermDbSchema.CourseTable.Cols.MENTOR_NAME + ", " +
            TermDbSchema.CourseTable.Cols.MENTOR_PHONE + ", " +
            TermDbSchema.CourseTable.Cols.MENTOR_EMAIL + ", " + //<<<<<<<<<< CHANGED
            TermDbSchema.CourseTable.Cols.TERM_LINK + " INTEGER REFERENCES " + TermDbSchema.TermTable.NAME + "(_id)" + //<<<<<<<<<< ADDED

            ")"
    );

• 您可能希望遵守约定并省略 INTEGER(列的类型亲和性)，这不是问题。之所以将其包含在内，是因为大多数人会指定列类型而不是应用默认值(数字)。

重要

然后重写onConfigure来调用setForeignKeyConstraintsEnabled 传递真实。

例如:-

@Override
public void onConfigure(SQLiteDatabase db) {
    super.onConfigure(db);
    db.setForeignKeyConstraintsEnabled(true);
}

• 默认情况下，外键支持处于关闭状态，除非打开外键支持，否则上述编码更改将毫无用处。

然后您需要执行以下操作之一:-

• 删除应用数据
• 卸载应用

然后重新运行应用程序。

• 注意任何现有数据都将丢失。如果您需要保留数据，那么这可能相对复杂。

外键

请注意，定义外键只是定义一个约束(规则)，该约束要求将值放入具有约束的列中，该列必须是引用的父表/列的某一行中的值。

定义外键约束不会自动使关系发生。也就是说，在添加类(class)时您仍然必须确定相关术语(一个常见的误解是它会这样做)。

您可能希望考虑扩展定义以包括 ON DELETE 和 ON UPDATE 操作，例如 CASCADE。例如ON DELETE CASCADE 会在删除学期行时删除类(class)表中的子行。同样，如果术语表中的值发生更改，ON UPDATE CASCADE 将更新类(class)表中子级的引用值(这些可以使生活更简单)。

例如你可以使用:-

    TermDbSchema.CourseTable.Cols.TERM_LINK + " INTEGER REFERENCES " + TermDbSchema.TermTable.NAME + "(_id) ON DELETE CASCADE ON UPDATE CASCADE" + //<<<<<<<<<< ADDED

您不妨引用SQLite Foreign Key Support