Get a pair of records from group by clause

Question

I have some tables from this table and I use this table to get records

string_agg(a2.sampl_no, ', ') as sampl_nos

The

and group by clauses work fine, but I want to have at most 4 sample_no aggregates per row.

Suppose I get

from the database

string_agg as (0001, 0002, 0003, 0004, 0005, 0006)

In a row, but I want this

(0001, 0002, 0003, 0004) (0006, 0007).

Help me solve this problem.

P粉564192131 · Answer

If you want to do this in the database, you can use row_number() to split the records into four groups and then use string_agg():

select * from rec_agg;
┌───────────┐
│ sample_no │
├───────────┤
│ 0001      │
│ 0002      │
│ 0003      │
│ 0004      │
│ 0005      │
│ 0006      │
│ 0007      │
│ 0008      │
│ 0009      │
│ 0010      │
│ 0011      │
└───────────┘
(11 rows)


with mk_grp as (
  select *, ((row_number() over (order by sample_no)) - 1) / 4 as grp 
    from rec_agg
) 
select string_agg(sample_no, ' ,') 
  from mk_grp 
 group by grp ;
┌────────────────────────┐
│       string_agg       │
├────────────────────────┤
│ 0001 ,0002 ,0003 ,0004 │
│ 0009 ,0010 ,0011       │
│ 0005 ,0006 ,0007 ,0008 │
└────────────────────────┘
(3 rows)