Question

我目前正在使用以下查询将两个表连接在一起：

select distinct book_id, clean_page_content as page_content, 'clean' as status, reg_date
from book_db_clean_pages t
where book_id = 'sarafi_book_5'
and   reg_date = '2015-08-26'

UNION all

select distinct book_id, dirty_page_content as page_content, 'dirty' as status, reg_date
from book_db_dirty_pages e
where book_id = 'sarafi_book_5'
and   reg_date = '2015-08-26'

但是，当我想要寻找不同的 book_id 时，我需要更改两个子句（一个在上面，一个在 UNION 下面）。同样的原则适用于 reg_date 。所以我想知道是否可以简单地使用像 join 这样的命令来简化我的查询。原则上，我正在寻找之类的解决方案：

select distinct t.book_id, (t.clean_page_content or e.dirty_page_content) as page_content, 
       ('clean' or 'dirty') as status, t.reg_date
from book_db_clean_pages t, book_db_dirty_pages e
where t.book_id = e.book_id and t.reg_date = e.reg_date
and t.book_id = 'sarafi_book_5'
and t.reg_date = '2015-08-26'

Answer 1

将你的联合放入派生表：

a href="{{ app.request.getSchemeAndHttpHost() }}/contact"

所有现代DBMS都足够聪明，可以将条件推送到派生表中，因此在您执行此操作时不会出现性能损失。

顺便说一句：如果select * from ( select ... from book_db_clean_pages t -- no where clause union all select ... from book_db_dirty_pages e where t.book_id = e.book_id and t.reg_date = e.reg_date -- no where clause for book_id and reg_date ) t where book_id = 'sarafi_book_5' and reg_date = '2015-08-26';是两个表中的主键（或唯一列），则在内部选择上应用book_id将不会删除任何内容。

Answer 2

您无法在编写查询时简化查询。

你可以写：

public void getDataBaseData() {
        // /PawnStorescopy.db
        if (!new File("/data/data/" + this.getPackageName()
                + "/PawnStorescopy.sqlite").exists()) {
            try {
                FileOutputStream out = new FileOutputStream("data/data/"
                        + this.getPackageName() + "DatabaseName.sqlite");
                InputStream in = getAssets().open("DatabaseName.db");
                byte[] buffer = new byte[1024];
                int readBytes = 0;

                while ((readBytes = in.read(buffer)) != -1)
                    out.write(buffer, 0, readBytes);

                in.close();
                // out.close();
            } catch (IOException e) {
            }
        }

        SQLiteDatabase sqliteDB = SQLiteDatabase.openOrCreateDatabase( 
                "/data/data/" + this.getPackageName()
                        + "/DatabaseName.sqlite", null);

        Cursor cursor = sqliteDB.rawQuery("SELECT * FROM TableName", null);

        if (cursor.moveToFirst()) {
            do {
                ////Get Data
            } while (cursor.moveToNext());


            imageFilter(listDatabase);
        }

    }

您没有提到您正在使用的数据库。如果子查询可以使用索引，那么初始查询可能会更有效率。

SQL - 简化UNION查询

2 个答案: