Notepad Tutorial（2）

 public long createNote(String title, String body) {
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_TITLE, title);
        initialValues.put(KEY_BODY, body);

        return mDb.insert(DATABASE_TABLE, null, initialValues);
    }

ContentValues  class is used to store a set of values that the  ContentResolver can process.这个类实现了Parcelable接口。

如上代码所示，ContentValues实际上存储了一些“键值对”，和 Hashtable比较类似，但是它存储的键值对当中的键是一个String类型，而值都是基本类型。

mDb是SQLiteDatabase类型，insert方法负责插入一个键值对。nullColumnHack：当values参数为空或者里面没有内容的时候，我们insert是会失败的（底层数据库不允许插入一个空行），为了防止这种情况，我们要在这里指定一个列名，到时候如果发现将要插入的行为空行时，就会将你指定的这个列名的值设为null，然后再向数据库中插入。

 SQL doesn't allow inserting a completely empty row without  naming at least one column name.  If your provided values is  empty, no column names are known and an empty row can't be inserted.   If not set to null, the nullColumnHack parameter  provides the name of nullable column name to explicitly insert a NULL into    in the case where your values is empty.

 public long insertWithOnConflict(String table, String nullColumnHack,
            ContentValues initialValues, int conflictAlgorithm) {
        StringBuilder sql = new StringBuilder();
        sql.append("INSERT");
        sql.append(CONFLICT_VALUES[conflictAlgorithm]);
        sql.append(" INTO ");
        sql.append(table);
        sql.append('(');

        Object[] bindArgs = null;
        int size = (initialValues != null && initialValues.size() > 0) ? initialValues.size() : 0;
        if (size > 0) {
            bindArgs = new Object[size];
            int i = 0;
            for (String colName : initialValues.keySet()) {
                sql.append((i > 0) ? "," : "");
                sql.append(colName);
                bindArgs[i++] = initialValues.get(colName);
            }
            sql.append(')');
            sql.append(" VALUES (");
            for (i = 0; i < size; i++) {
                sql.append((i > 0) ? ",?" : "?");
            }
        } else {
            sql.append(nullColumnHack + ") VALUES (NULL");
        }
        sql.append(')');

        SQLiteStatement statement = new SQLiteStatement(this, sql.toString(), bindArgs);
        try {
            return statement.executeInsert();
        } catch (SQLiteDatabaseCorruptException e) {
            onCorruption();
            throw e;
        } finally {
            statement.close();
        }
    }

 insert方法会调用上述方法，注意看size

当我们的ContentValues类型的数据initialValues为null，或者size<=0时，就会再sql语句中添加nullColumnHack的设置。我们可以想象一下，如果我们不添加nullColumnHack的话，那么我们的sql语句最终的结果将会类似insert into tableName()values();这显然是不允许的。而如果我们添加上nullColumnHack呢，sql将会变成这样，insert into tableName (nullColumnHack)values(null);这样很显然就是可以的。

    public boolean deleteNote(long rowId) {

        return mDb.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
    }

 删除方法：

public int delete(String table, String whereClause, String[] whereArgs) {
        SQLiteStatement statement =  new SQLiteStatement(this, "DELETE FROM " + table +
                (!TextUtils.isEmpty(whereClause) ? " WHERE " + whereClause : ""), whereArgs);
        try {
            return statement.executeUpdateDelete();
        } catch (SQLiteDatabaseCorruptException e) {
            onCorruption();
            throw e;
        } finally {
            statement.close();
        }
    }

 whereClause：条件语句,注意这里的条件语句并不包括where这个单词。例如 name="chenzheng_java" and age=23

 whereArgs：我们再where子句中，为了防止sql注入，我们通常会通过？的方式进行代替，然后再为？赋值。

 当删除正常执行时，返回的是删除的数据的个数；否则的话，返回0.如果我们想全部删除的话，whereClause设置为“1”即可。

 查询方法：

public Cursor fetchAllNotes() {

        return mDb.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_TITLE,
                KEY_BODY}, null, null, null, null, null);
    }

 public Cursor fetchNote(long rowId) throws SQLException {

        Cursor mCursor =

            mDb.query(true, DATABASE_TABLE, new String[] {KEY_ROWID,
                    KEY_TITLE, KEY_BODY}, KEY_ROWID + "=" + rowId, null,
                    null, null, null, null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;

    }

public Cursor query (String table, String[] columns, String selection, String[] selectionArgs, String groupBy, String having, String orderBy)
Parameters
table  The table name to compile the query against.
columns  A list of which columns to return. Passing null will return all columns, which is discouraged to prevent reading data from storage that isn't going to be used.
selection  A filter declaring which rows to return, formatted as an SQL WHERE clause (excluding the WHERE itself). Passing null will return all rows for the given table.
selectionArgs  You may include ?s in selection, which will be replaced by the values from selectionArgs, in order that they appear in the selection. The values will be bound as Strings.
groupBy  A filter declaring how to group rows, formatted as an SQL GROUP BY clause (excluding the GROUP BY itself). Passing null will cause the rows to not be grouped.
having  A filter declare which row groups to include in the cursor, if row grouping is being used, formatted as an SQL HAVING clause (excluding the HAVING itself). Passing null will cause all row groups to be included, and is required when row grouping is not being used.
orderBy  How to order the rows, formatted as an SQL ORDER BY clause (excluding the ORDER BY itself). Passing null will use the default sort order, which may be unordered.