[英]Why doesn't my SQL query select any row from the sqllite database
在我的数据库中，我有两个表“ classes”和“ classes_sessions”，我创建了一个链接这两个表的外键。 我正在尝试使用以下查询从这些表中检索数据
码
String selectQuery = "SELECT * FROM " +
TABLE_CLASSES +
" INNER JOIN " + TABLE_CLASSES_SECTIONS +
" ON " +
TABLE_CLASSES_SECTIONS + "." + COLUMN_CLASSES_SECTIONS_ID +
" = " + TABLE_CLASSES + "." + COLUMN_CLASSES_SECTIONS +
" WHERE " +
TABLE_CLASSES + "." + COLUMN_CLASSES_ID +
" = " + String.valueOf(id);
在下面的getAllSectionsByClassesID()方法中
public ArrayList<SectionsBean> getAllSectionsByClassesID(long id){
String selectQuery = "SELECT * FROM " +
TABLE_CLASSES +
" INNER JOIN " + TABLE_CLASSES_SECTIONS +
" ON " +
TABLE_CLASSES_SECTIONS + "." + COLUMN_CLASSES_SECTIONS_ID +
" = " + TABLE_CLASSES + "." + COLUMN_CLASSES_SECTIONS +
" WHERE " +
TABLE_CLASSES + "." + COLUMN_CLASSES_ID +
" = " + String.valueOf(id);
SQLiteDatabase db = this.getReadableDatabase();
ArrayList<SectionsBean> sectionsBeanList = new ArrayList<SectionsBean>();
Cursor cursor = db.rawQuery(selectQuery,null);
Log.i("Query details", String.valueOf(cursor));
Log.d("DataDetails", DatabaseUtils.dumpCursorToString(cursor));
while (cursor.moveToNext()) {
ClassesBean classesBean = new ClassesBean();
classesBean.setId(cursor.getLong(cursor.getColumnIndex(COLUMN_CLASSES_ID)));
classesBean.setClasses_name(cursor.getString(cursor.getColumnIndex(COLUMN_CLASSES_NAME)));
SectionsBean sectionsBean = new SectionsBean();
sectionsBean.setSectionsID(cursor.getLong(cursor.getColumnIndex(COLUMN_CLASSES_SECTIONS_ID)));
sectionsBean.setSections_name(cursor.getString(cursor.getColumnIndex(COLUMN_CLASSES_SECTIONS_NAME)));
sectionsBean.setClassesBean(classesBean);
sectionsBeanList.add(sectionsBean);
}
return sectionsBeanList;
}
但是它不返回任何东西。 我正在使用该行来检查数据库Log.d("DataDetails", DatabaseUtils.dumpCursorToString(cursor));光标返回的数据Log.d("DataDetails", DatabaseUtils.dumpCursorToString(cursor)); 结果为空白。 这两个表中都有内容，如下所示
数据库内容
班级表
-05 22:06:23.728 31258-31310/com.example.demeainc.demea D/DataC: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@88a8264
0 {
classes_id=1
class_item_index=null
classes_name=jss1
classes_codename=null
classes_sections_id=null
classes_teachers=null
classes_students=null
}
1 {
classes_id=2
class_item_index=null
classes_name=villa
classes_codename=null
classes_sections_id=null
classes_teachers=null
classes_students=null
}
2 {
classes_id=3
class_item_index=null
classes_name=two
classes_codename=null
classes_sections_id=null
classes_teachers=null
classes_students=null
}
<<<<<
会话表的数据库内容
03-05 22:09:03.943 31258-31258/com.example.demeainc.demea D/DataS: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@eaadf23
0 {
classes_sections_ids=1
classes_sections_name=cooolanet
classes_sections_description=bbdbn
}
1 {
classes_sections_ids=2
classes_sections_name=morrals
classes_sections_description=mills
}
2 {
classes_sections_ids=3
classes_sections_name=live
classes_sections_description=bxn
}
3 {
classes_sections_ids=4
classes_sections_name=testing2
classes_sections_description=coll
}
4 {
classes_sections_ids=5
classes_sections_name=tool
classes_sections_description=vi
}
5 {
classes_sections_ids=6
classes_sections_name=colls
classes_sections_description=
}
6 {
classes_sections_ids=7
classes_sections_name=more
classes_sections_description=coll
}
7 {
classes_sections_ids=8
classes_sections_name=testing
classes_sections_description=ttt
}
8 {
classes_sections_ids=9
classes_sections_name=threevill
classes_sections_description=cool
}
<<<<<
有关表创建的更多详细信息。
// create classes_table sql query
private String CREATE_CLASSES_TABLE = "CREATE TABLE IF NOT EXISTS " + TABLE_CLASSES + "("
+ COLUMN_CLASSES_ID + " INTEGER PRIMARY KEY AUTOINCREMENT," + COLUMN_CLASS_ITEM_INDEX + " NUMBER,"
+ COLUMN_CLASSES_NAME + " VARCHAR," + COLUMN_CLASSES_CODENAME + " VARCHAR, " + COLUMN_CLASSES_SECTIONS + " INTEGER," + COLUMN_CLASSES_TEACHERS
+ " VARCHAR," + COLUMN_CLASSES_STUDENTS + " VARCHAR,"
+ "FOREIGN KEY(" + COLUMN_CLASSES_SECTIONS + ") REFERENCES " + TABLE_CLASSES_SECTIONS + "(" + COLUMN_CLASSES_SECTIONS_ID + ") ON DELETE CASCADE " + ");";
//create sections sql query
private String CREATE_CLASSES_SECTIONS_TABLE = "CREATE TABLE IF NOT EXISTS " + TABLE_CLASSES_SECTIONS + "("
+ COLUMN_CLASSES_SECTIONS_ID + " INTEGER PRIMARY KEY AUTOINCREMENT," + COLUMN_CLASSES_SECTIONS_NAME + " VARCHAR,"
+ COLUMN_CLASSES_SECTIONS_DESCRIPTION + " VARCHAR" + ")";
可能是什么问题。 外键实际上是链接两个表吗？ 我在上面的代码中出错了。
您的问题是class_sections_id列为 null，因此在classes_sections_table（会话表）中永远不会有匹配的条目，因此没有进行JOIN操作，因此无任何显示。
下面的类表作为示例说明如何工作：-
现在classes_sections表：-
对于英语，您将看到classes_sections_id的值为1，它可以对应于（值为/与其关联/引用等）值为1的classes_sections_ids列。
查询
SELECT * FROM classes
INNER JOIN classes_sections
ON classes_sections.classes_sections_ids = classes.classes_sections_id
WHERE classes.classes_id =1
会导致：-
但是，如果将以下行添加到classes表中（ 请注意，对classes_sections_id列的引用的值为700，并且classes_sections表中没有此类行 ）：
如果将查询更改为获取ID 4，则不会返回任何行，因为即使在类表中确实存在ID 4（化学），也没有关联的classes_sections行（ classes_sections_ids为700 ），因此没有JOIN，因此没有行返回。
简而言之，您需要sections_id列引用/关联/链接到classes_sections_ids列以检索任何数据。
外键实际上是链接两个表吗？
指定FOREIGN KEY仅指定存在约束（请注意ON DELETE CASCADE限制该约束）。 它不会自动定义链接，您必须执行此操作。
您可能会以编程方式执行此操作（我手动创建了以上链接），例如，添加类时，您将选择一个可用的部分，也许是通过微调器（又名下拉选择器）显示的。 然后，插入具有所选部分的ID的类。
我相信您最终将在设计上遇到问题，这主要是由于您的引用/链接方式所致。
例如，您的设计有一个class表，其中包含一列以链接到sections表。 这样就不会引入在单个列中处理链接列表的复杂性，从而限制了班级只有一个部分（教师和学生也是如此）。
在一个班级可能有多个部分的假设的基础上，还有一个进一步的考虑，一个班级可以被许多班级使用（一个例子是，所有班级都必须以疏散程序开始）。
如果后者不适用，则类和节之间的关系可能是一对多的（或可能由多对多的关系处理）。
如果后者适用，则类与节之间的关系将/应该是多对多关系。
因此，我建议您考虑以下设计（也许还会有更多设计）：
DROP TABLE IF EXISTS classes;
CREATE TABLE IF NOT EXISTS classes ( class_id INTEGER PRIMARY KEY, class_name TEXT, class_codename TEXT );
DROP TABLE IF EXISTS sections;
CREATE TABLE IF NOT EXISTS sections ( section_id INTEGER PRIMARY KEY, section_name TEXT, section_description TEXT);
DROP TABLE IF EXISTS teachers;
CREATE TABLE IF NOT EXISTS teachers ( teacher_id INTEGER PRIMARY KEY, teacher_name TEXT);
DROP TABLE IF EXISTS students;
CREATE TABLE IF NOT EXISTS students ( student_id INTEGER PRIMARY KEY, student_name TEXT);
-- LINK TABLES
-- Note usess column constraints to define foreign keys i.e. REFERENCES
DROP TABLE IF EXISTS class_section_links;
CREATE TABLE IF NOT EXISTS class_section_links (
class_link INTEGER NOT NULL REFERENCES classes (class_id),
section_link INTEGER NOT NULL REFERENCES sections (section_id),
PRIMARY KEY (class_link, section_link));
DROP TABLE IF EXISTS class_teacher_links;
CREATE TABLE IF NOT EXISTS class_teacher_links (
class_link INTEGER NOT NULL REFERENCES classes (class_id),
teacher_link INTEGER NOT NULL REFERENCES teachers (teacher_id),
PRIMARY KEY (class_link, teacher_link));
DROP TABLE IF EXISTS class_student_links;
CREATE TABLE IF NOT EXISTS class_student_links (
class_link INTEGER NOT NULL REFERENCES classes (class_id),
student_link INTEGER NOT NULL REFERENCES students (student_id),
PRIMARY KEY (class_link, student_link));
这将加载一些数据，包括类和节之间的一些基本链接：
-- LOAD some data
-- CLASSES
INSERT INTO classes (class_name, class_codename) VALUES('English Language','EL100');
INSERT INTO classes (class_name, class_codename) VALUES('English Literature','EL101');
INSERT INTO classes (class_name, class_codename) VALUES('Applied Mathermatics','MA200');
INSERT INTO classes (class_name, class_codename) VALUES('Pure Mathematics','MA201');
INSERT INTO classes (class_name, class_codename) VALUES('Chemistry','SC300');
INSERT INTO classes (class_name, class_codename) VALUES('Physics','SC301');
INSERT INTO classes (class_name, class_codename) VALUES('Biology','SC302');
INSERT INTO classes (class_name, class_codename) VALUES('GEOGRAPHY','GE400');
-- SECTIONS
INSERT INTO sections (section_name, section_description) VALUES('Class Introduction','Evacuation procedures, amenities etc..');
INSERT INTO sections (section_name, section_description) VALUES('Sentence Construction','Blah');
INSERT INTO sections (section_name, section_description) VALUES('Word types','Basic word types such as VERB, ADJECTIVE etc');
INSERT INTO sections (section_name, section_description) VALUES('Puntuation','Blah');
INSERT INTO sections (section_name, section_description) VALUES('Under Milk Wood','Blah');
INSERT INTO sections (section_name, section_description) VALUES('Catcher in the Rye','Blah');
INSERT INTO sections (section_name, section_description) VALUES('The War of the Worlds','Blah');
-- CLASS SECTION LINKS (note assumes ID's of classes/sections in order 1,2,3......)
-- a) All classes have Class Introduction
INSERT INTO class_section_links VALUES(1,1); -- Class 1 English language
INSERT INTO class_section_links VALUES(2,1); -- Class 2 English Lit
INSERT INTO class_section_links VALUES(3,1);
INSERT INTO class_section_links VALUES(4,1);
INSERT INTO class_section_links VALUES(5,1);
INSERT INTO class_section_links VALUES(6,1);
INSERT INTO class_section_links VALUES(7,1);
INSERT INTO class_section_links VALUES(8,1);
-- b) specific sections
INSERT INTO class_section_links VALUES(1,2); -- Class 1 has section 2
INSERT INTO class_section_links VALUES(1,3); -- Class 1 has section 3
INSERT INTO class_section_links VALUES(2,4);
INSERT INTO class_section_links VALUES(2,5);
INSERT INTO class_section_links VALUES(2,6);
INSERT INTO class_section_links VALUES(2,7);
查询，例如：
SELECT class_name, class_codename, section_name, section_description
FROM class_section_links
JOIN classes ON class_link = class_id
JOIN sections ON section_link = section_id
ORDER BY class_name, section_name;
会导致：-
如果查看“ 类简介”，您将看到仅一次使用了一个部分，因此只需要一组数据。 因此，如果有，与其类介绍将标题改为介绍接单的变化将更新所有类一个指令。
例如，使用以下命令执行更新：
UPDATE sections SET section_name = 'Introduction' WHERE section_name = 'Class Introduction';
并运行相同的查询结果：
尽管这不是对问题的答案，但可能会有用。
这是一个快速汇总的演示，演示如何在App中实现链接表。 请注意，这非常基础。
该应用程序最初将允许输入一个类 （要求两个字段都具有数据），单击“ 添加类 ”按钮将尝试添加一个类（如果两个输入均至少包含一个字符）。
添加一个类后，将提供三个附加输入（使它们可见），并且还将列出添加的类。 新的输入是：
添加部分后，该部分将在右侧列出，此外，组合的链接数据将在列和部分列表下方列出。
例如
（请注意，包名称必须更改）
<TextView android:id="@+id/heading" android:layout_width="match_parent" android:layout_height="wrap_content" /> <EditText android:id="@+id/class_name" android:layout_width="wrap_content" android:layout_height="wrap_content" /> <EditText android:id="@+id/class_codename" android:layout_width="wrap_content" android:layout_height="wrap_content" /> <Button android:id="@+id/addclass" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Add Class"/> <EditText android:id="@+id/section_name" android:layout_width="wrap_content" android:layout_height="wrap_content" /> <EditText android:id="@+id/section_description" android:layout_width="wrap_content" android:layout_height="wrap_content" /> <Spinner android:id="@+id/classselection" android:layout_width="wrap_content" android:layout_height="wrap_content"> </Spinner> <Button android:id="@+id/addsection" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Add Section"/> <LinearLayout android:orientation="horizontal" android:id="@+id/lists" android:layout_width="match_parent" android:layout_height="wrap_content"> <ListView android:id="@+id/classlist" android:layout_width="0dp" android:layout_weight="1" android:layout_height="wrap_content"> </ListView> <ListView android:id="@+id/sectionlist" android:layout_width="0dp" android:layout_weight="1" android:layout_height="wrap_content"></ListView> </LinearLayout> <ListView android:id="@+id/classsectionsinfo" android:layout_width="wrap_content" android:layout_height="wrap_content"> </ListView> public class DBHelper extends SQLiteOpenHelper {
public static final String DBNAME = "educator";
public static final int DBVERSION = 1;
public static final String TB_CLASSES = "classes";
public static final String TB_SECTIONS = "sections";
public static final String TB_CLASS_SECTION_LINKS = "class_section_links";
public static final String COL_CLASSID = BaseColumns._ID;
public static final String COl_CLASSNAME = "class_name";
public static final String COl_CLASSCODENAME = "class_codename";
public static final String COL_SECTIONID = BaseColumns._ID;
public static final String COL_SECTIONNAME = "section_name";
public static final String COL_SECTIONDESCRIPTION = "section_description";
public static final String COL_CLASSLINK = "class_link";
public static final String COL_SECTIONLINK = "section_link";
public static final String COL_COMBINED = "info";
SQLiteDatabase mDB;
public DBHelper(Context context) {
super(context, DBNAME, null, DBVERSION);
mDB = this.getWritableDatabase();
}
@Override
public void onCreate(SQLiteDatabase db) {
String crttab = "CREATE TABLE IF NOT EXISTS ";
String crtclasses = crttab + TB_CLASSES +
"(" +
COL_CLASSID + " INTEGER PRIMARY KEY, " +
COl_CLASSNAME + " TEXT, " +
COl_CLASSCODENAME + " TEXT " +
")";
String crtsections = crttab + TB_SECTIONS +
"(" +
COL_SECTIONID + " INTEGER PRIMARY KEY, " +
COL_SECTIONNAME + " TEXT, " +
COL_SECTIONDESCRIPTION + " TEXT " +
")";
String crtclasssectionlink = crttab + TB_CLASS_SECTION_LINKS +
"(" +
COL_CLASSLINK + " INTEGER " +
" REFERENCES " + TB_CLASSES + "(" + COL_CLASSID + ")," +
COL_SECTIONLINK + " INTEGER " +
" REFERENCES " + TB_SECTIONS + "(" + COL_SECTIONID + ") " +
")";
db.execSQL(crtclasses);
db.execSQL(crtsections);
db.execSQL(crtclasssectionlink);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
public long addClass(String classname, String classcode) {
if (classname.length() < 1 || classcode.length() < 1) {
return -1;
}
ContentValues cv = new ContentValues();
cv.put(COl_CLASSNAME,classname);
cv.put(COl_CLASSCODENAME,classcode);
return mDB.insert(TB_CLASSES,null, cv);
}
public long addSection(String sectioname, String sectiondescription, long baseclass) {
long sectionid = -1;
if (sectioname.length() < 1 || sectiondescription.length() < 1) {
return -1;
}
ContentValues cv = new ContentValues();
cv.put(COL_SECTIONNAME,sectioname);
cv.put(COL_SECTIONDESCRIPTION,sectiondescription);
sectionid = mDB.insert(TB_SECTIONS,null,cv);
if (sectionid > 0 && baseclass > 0) {
cv.clear();
cv.put(COL_CLASSLINK,baseclass);
cv.put(COL_SECTIONLINK,sectionid);
mDB.insert(TB_CLASS_SECTION_LINKS,null,cv);
}
return sectionid;
}
public Cursor getClassAndSectionDetailsCombined() {
//SELECT class_name||class_codename||section_name||section_description AS info
// FROM class_section_links
// JOIN classes ON class_link = classes._id
// JOIN sections ON section_link = sections._id
// ORDER BY class_name, section_name;
return mDB.query(
TB_CLASS_SECTION_LINKS +
" JOIN " + TB_CLASSES + " ON " +
COL_CLASSLINK + " = " + TB_CLASSES + "." + COL_CLASSID +
" JOIN " + TB_SECTIONS + " ON " +
COL_SECTIONLINK + " = " + TB_SECTIONS + "." + COL_SECTIONID,
new String[] {
COl_CLASSNAME + "||" +
COl_CLASSCODENAME + "||" +
COL_SECTIONNAME + "||" +
COL_SECTIONDESCRIPTION +
" AS " + COL_COMBINED,
"1 AS " + BaseColumns._ID},
null,null,null, null,
COl_CLASSNAME + "," + COL_SECTIONNAME
);
}
public Cursor getClasses() {
return mDB.query(TB_CLASSES,null,null,null,null,null,null);
}
public Cursor getSections() {
return mDB.query(TB_SECTIONS,null,null,null,null,null,null);
}
public long getSectionsCount() {
return DatabaseUtils.queryNumEntries(mDB,TB_SECTIONS);
}
public long getClassesCount() {
return DatabaseUtils.queryNumEntries(mDB,TB_CLASSES);
}
}
getClassAndSectionDetailsCombined与注释的SQL有所不同，因为游标具有两列，其中一列是_id ，这是为了方便使用需要_id列的CursorAdapters。
public class MainActivity extends AppCompatActivity {
EditText mClassName, mClassCode, mSectionName, mSectionDescription;
Button mAddClass, mAddSection;
ListView mClassList, mSectionList, mClassSectionInfoList;
Spinner mClassSelection;
DBHelper mDBHlpr;
Cursor mClasses, mSections, mCLassSectionInfo;
SimpleCursorAdapter mSCAClasses, mSCASections, mSCAClassSecInfo;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mClassName = (EditText) this.findViewById(R.id.class_name);
mClassCode = (EditText) this.findViewById(R.id.class_codename);
mSectionName = (EditText) this.findViewById(R.id.section_name);
mSectionDescription = (EditText) this.findViewById(R.id.section_description);
mAddClass = (Button) this.findViewById(R.id.addclass);
mAddSection = (Button) this.findViewById(R.id.addsection);
mClassList = (ListView) this.findViewById(R.id.classlist);
mSectionList = (ListView) this.findViewById(R.id.sectionlist);
mClassSectionInfoList = (ListView) this.findViewById(R.id.classsectionsinfo);
mClassSelection = (Spinner) this.findViewById(R.id.classselection);
mDBHlpr = new DBHelper(this);
refreshDisplay();
handleButtons();
}
private void handleButtons() {
mAddClass.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if ((mClassName.getText().toString().length() > 0) &&
mClassCode.getText().toString().length() > 0) {
mDBHlpr.addClass(
mClassName.getText().toString(),
mClassCode.getText().toString()
);
refreshDisplay();
}
}
});
mAddSection.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if ((mSectionName.getText().toString().length() > 0) &&
(mSectionDescription.getText().toString().length() > 0)) {
mDBHlpr.addSection(
mSectionName.getText().toString(),
mSectionDescription.getText().toString(),
mClassSelection.getSelectedItemId()
);
refreshDisplay();
}
}
});
}
private void refreshDisplay() {
// Only allow sections to be added if at least one Class exists
if (mDBHlpr.getClassesCount() < 1) {
mSectionName.setVisibility(View.GONE);
mSectionDescription.setVisibility(View.GONE);
mAddSection.setVisibility(View.GONE);
} else {
mSectionName.setVisibility(View.VISIBLE);
mSectionDescription.setVisibility(View.VISIBLE);
mAddSection.setVisibility(View.VISIBLE);
}
// Get Cursors from DB
mClasses = mDBHlpr.getClasses();
mSections = mDBHlpr.getSections();
mCLassSectionInfo = mDBHlpr.getClassAndSectionDetailsCombined();
// Prepare the Classes List Adapter or swap the cursor
if (mSCAClasses == null) {
mSCAClasses = new SimpleCursorAdapter(this,
android.R.layout.simple_list_item_1,
mClasses,new String[]{DBHelper.COl_CLASSNAME},
new int[]{android.R.id.text1},
0
);
} else {
mSCAClasses.swapCursor(mClasses);
}
// Prepare the Sections List Adapter
if (mSCASections == null) {
mSCASections = new SimpleCursorAdapter(this,
android.R.layout.simple_list_item_1,
mSections,
new String[]{DBHelper.COL_SECTIONNAME},
new int[]{android.R.id.text1},
0
);
} else {
mSCASections.swapCursor(mSections);
}
if (mSCAClassSecInfo == null) {
mSCAClassSecInfo = new SimpleCursorAdapter(this,
android.R.layout.simple_list_item_1,
mCLassSectionInfo,
new String[]{DBHelper.COL_COMBINED},
new int[]{android.R.id.text1},
0
);
} else {
mSCAClassSecInfo.swapCursor(mCLassSectionInfo);
}
mClassList.setAdapter(mSCAClasses);
mClassList.setBackgroundColor(0xFF5555ff);
mSectionList.setAdapter(mSCASections);
mSectionList.setBackgroundColor(0xFF55FF55);
mClassSelection.setAdapter(mSCAClasses);
mClassSectionInfoList.setAdapter(mSCAClassSecInfo);
mClassSectionInfoList.setBackgroundColor(0xFFFFFFDD);
}
}
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