问题
I am trying to convert a query which returns a single row with multiple columns to multiple rows with one or multiple columns. As an example my query looks like the following visualized:
SELECT _id, value1, value2, value3, value4 FROM table WHERE _id IS 1;
RESULT:
_id value1 value2 value3 value4
----------------------------------------
1 10 11 12 13
I would like to have my results look like the following:
name value
--------------
value1 10
value2 11
value3 12
value4 13
but I can also work with something like this:
value
-----
10
11
12
13
Currently I'm manually converting this using a MatrixCursor in Android, but the ideal situation would be to have the cursor already in a usable state. Is there a way to either modify my query to retrieve the results in my desired schema or to use something like a temp table to achieve the desired end state?
Also, is there a common term to describe this functionality within a Database?
回答1:
There is no built-in mechanism that could do this automatically.
If you know the columns, you can construct a query that handles each column individually:
SELECT _id, 'value1' AS name, value1 AS value FROM tab WHERE _id = 1
UNION ALL
SELECT _id, 'value2' AS name, value2 AS value FROM tab WHERE _id = 1
UNION ALL
SELECT _id, 'value3' AS name, value3 AS value FROM tab WHERE _id = 1
UNION ALL
SELECT _id, 'value4' AS name, value4 AS value FROM tab WHERE _id = 1;
This is not necessarily any better than a MatrixCursor.
回答2:
Why not just JOIN on a values table and then CASE the correct column?
SELECT original_table._id, CASE t.column1 WHEN 1 THEN 'value1' WHEN 2 THEN 'value2' WHEN 3 THEN 'value3' ELSE 'value4' END AS name, CASE t.column1 WHEN 1 THEN original_table.value1 WHEN 2 THEN original_table.value2 WHEN 3 THEN original_table.value3 ELSE original_table.value4 END AS value
FROM (
select 1 _id, 10 value1 , 11 value2 , "string value" value3 , 13 value4
union all select 2, 14, "string value",null,17
) original_table
JOIN (VALUES(1),(2),(3),(4)) t
--ORDER BY _id, t.column1;
The result:
id name value
1 value1 10
1 value2 11
1 value3 string value
1 value4 13
2 value1 14
2 value2 string value
2 value3 {null}
2 value4 17
Alternatively add
ON (t.column1=1 AND original_table.value1 IS NOT NULL) OR (t.column1=2 AND original_table.value2 IS NOT NULL) OR (t.column1=3 AND original_table.value3 IS NOT NULL) OR (t.column1=4 AND original_table.value4 IS NOT NULL)
来源：https://stackoverflow.com/questions/42780565/turn-sqlite-columns-to-rows