Purposely having a query return blank entries at regular intervals - sql

Question

I want to write a query that returns 3 results followed by blank results followed by the next 3 results, and so on. So if my database had this data:
CREATE TABLE table (a integer, b integer, c integer, d integer);
INSERT INTO table (a,b,c,d)
VALUES (1,2,3,4),
(5,6,7,8),
(9,10,11,12),
(13,14,15,16),
(17,18,19,20),
(21,22,23,24),
(25,26,37,28);
I would want my query to return this
1,2,3,4
5,6,7,8
9,10,11,12
, , ,
13,14,15,16
17,18,19,20
21,22,23,24
, , ,
25,26,27,28
I need this to work for arbitrarily many entries that I select for, have three be grouped together like this.
I'm running postgresql 8.3

Answer 1

This should work flawlessly in PostgreSQL 8.3
SELECT a, b, c, d
FROM (
SELECT rn, 0 AS rk, (x[rn]).*
FROM (
SELECT x, generate_series(1, array_upper(x, 1)) AS rn
FROM (SELECT ARRAY(SELECT tbl FROM tbl) AS x) x
) y
UNION ALL
SELECT generate_series(3, (SELECT count(*) FROM tbl), 3), 1, (NULL::tbl).*
ORDER BY rn, rk
) z
Major points
Works for a query that selects all columns of tbl.
Works for any table.
For selecting arbitrary columns you have to substitute (NULL::tbl).* with a matching number of NULL columns in the second query.
Assuming that NULL values are ok for "blank" rows.
If not, you'll have to cast your columns to text in the first and substitute '' for NULL in the second SELECT.
Query will be slow with very big tables.
If I had to do it, I would write a plpgsql function that loops through the results and inserts the blank rows. But you mentioned you had no direct access to the db ...

Answer 2

In short, no, there's not an easy way to do this, and generally, you shouldn't try. The database is concerned with what your data actually is, not how it's going to be displayed. It's not an appropriate scope of responsibility to expect your database to return "dummy" or "extra" data so that some down-stream process produces a desired output. The generating script needs to do that.
As you can't change your down-stream process, you could (read that with a significant degree of skepticism and disdain) add things like this:
Select Top 3
a, b, c, d
From
table
Union Select Top 1
'', '', '', ''
From
table
Union Select Top 3 Skip 3
a, b, c, d
From
table
Please, don't actually try do that.

Answer 3

You can do it (at least on DB2 - there doesn't appear to be equivalent functionality for your version of PostgreSQL).
No looping needed, although there is a bit of trickery involved...
Please note that though this works, it's really best to change your display code.
Statement requires CTEs (although that can be re-written to use other table references), and OLAP functions (I guess you could re-write it to count() previous rows in a subquery, but...).
WITH dataList (rowNum, dataColumn) as (SELECT CAST(CAST(:interval as REAL) /
(:interval - 1) * ROW_NUMBER() OVER(ORDER BY dataColumn) as INTEGER),
dataColumn
FROM dataTable),
blankIncluder(rowNum, dataColumn) as (SELECT rowNum, dataColumn
FROM dataList
UNION ALL
SELECT rowNum - 1, :blankDataColumn
FROM dataList
WHERE MOD(rowNum - 1, :interval) = 0
AND rowNum > :interval)
SELECT *
FROM dataList
ORDER BY rowNum
This will generate a list of those elements from the datatable, with a 'blank' line every interval lines, as ordered by the initial query. The result set only has 'blank' lines between existing lines - there are no 'blank' lines on the ends.