如何从一个使用SQL查询独立排序的表中合并两个结果集？

Question

This is a simplified task which I have to solve in real project. In this project data is stored in HSQLDB. Data is accessed using JDBC.

I have one table:

name | flag
-----------
aa   | 1
bb   | 0
cc   | 1
dd   | 0
ee   | 1
ff   | 0

I need to compose query to get the following table:

name | flag
-----------
aa   | 1
cc   | 1
ee   | 1
ff   | 0
dd   | 0
bb   | 0

The final table is like rows with flag = 1 were taken and sorted ascending , rows with flag = 0 were taken and sorted descending and results were combined one after another.

Please, pay attention, that rows with flag = 1 and rows with flag = 0 have opposite sort order.

Is it possible to do in SQL? I wouldn`t like to make two queries and merge ResultSets in Java code manually.

Answer 1

In any SQL, only the outermost order by applies so it has to be done there.

I don't know if this works in your SQL dialect (and can't check sorry), but you could do this

SELECT name, flag
FROM  'table'
ORDER BY
    flag desc,
    CASE WHEN flag = 1 THEN name ELSE '' END,
    CASE WHEN flag = 0 THEN name ELSE '' END DESC

Answer 2

Try this:

SELECT name, flag
FROM 'table'
ORDER BY flag desc, name

Let the database do the work whenever you can. Don't do such things in Java. Think "SQL first".

Answer 3

order by can take more than one column:

select *
from table
order by flag desc, name asc