Question

I have a SQLite table ‘Details’ with structure:

ID  Name    Category   
---------------------
1   Matt    0 
2   Shervin 0 
3   Bob     0   
4   Lee     0 
5   Rick    0   
6   Suraya  0 
7   Susan   0   
8   Adam    0   
9   Jon     1   
10  Lorna   1 
... and so on .......  

I want to select a row at random, and then three names from three different rows (again preferably at random). I would like this to all be returned from one SQLite statement. E.g.

ID  Name  Category  Name1  Name2  Name 3  
----------------------------------------
3   Bob   0         Matt   Lee    Susan 

My attempt at this can be seen below, but it has two problems:

1. The three extra names are not necessarily always different – I can’t seem to exclude a name that has been previously selected because variables b/c/d are not in scope apart from their own COALESCE function.
2. As each nested select uses the Random() function it’s not very efficient.

Can anyone suggest another way to select the data I need (using SQLite database)? Any help/advice is welcome – hope it is clear what I am trying to achieve, feel free to ask for any clarifications.

My current attempt:

SELECT a.Id,
       a.Name,
       a.Category,
       COALESCE((SELECT b.Name 
                   FROM Details b 
                  WHERE b.Id NOT IN (a.Id)
                    AND b.Category IN (0)
               ORDER BY Random()
                  LIMIT 1),'') as "Name1",
       COALESCE((SELECT c.Name 
                   FROM Details c 
                  WHERE c.Id NOT IN (a.Id)
                    AND c.Category IN (0)
               ORDER BY Random()
                  LIMIT 1),'') as "Name2",
       COALESCE((SELECT d.Name 
                   FROM Details d
                  WHERE d.Id NOT IN (a.Id)
                    AND d.Category IN (0)
               ORDER BY Random()
                  LIMIT 1),'') as "Name3"
    FROM Details a
     AND a.Category IN (0)
ORDER BY Random()
   LIMIT 1

Answer 1

I’m with neurino here. You have not said why you need to put the four names chosen at random into a single row, and why this has to be done in the back-end.

If you’re concerned about performance, generate random integers in your client (range >= min(pkcol) and <= max(pkcol) ) until you’ve found four distinct rows (i.e. entities/names). There’s a chance that no row exists with one of the generated ids, but that takes mere milliseconds to find out. Taking that random-key approach you could avoid an order by. The approach would work quickly even for tables with billions of rows.

P.S. (After finding out it was an iPhone app)
You need one call to get the min and max ID values (it’s the PK so that uses an index). Then you need at least another call to the DB (again, index-assisted) to get the four distinct rows using your randomly generated PK values [where ID in (a, b, c , d) ] The maximum number of calls is unknown; how many will depend on the density of your primary key sequence. I do not believe this would be an inordinate amount of I/O and it would be considerably less resource-intensive than an order by Random()–especially if the table has many rows. You could always generate an ID list of 8, 12, 16 ids at random and have your client cull only the 4 rows required if more than 4 are returned.

P.P.S. Typically it is the instantiation of the database connection that is expensive, and you don’t want to do that in a loop or any more often than you need to. But you can open a connection, run two or three efficient selects that return a few rows each, and then close if you’re done with the task at hand.