Re: Use of array_agg and array string on inner query

The original table is :

c1 c2 c3
1    10
2    20 10
3    20 10

So c3 of row 3 and row 2 are equal to c2 of row 1.


The output I am looking for is :
 c1 | array_to_string
     ----+-----------------
       1 | 2,3
       2 |
       3 |
      (3 rows)

How Can I modify this query :

SELECT c1, c2,
    ARRAY_TO_STRING(ARRAY_AGG((SELECT t3.c1 FROM s.t1 as t3 JOIN s.t1
as t2 ON t3.c3 = t2.c2)), ',')
    FROM s.t1 t1
    GROUP BY c1;
    DROP SCHEMA s CASCADE;

to get me the output desired.

Thanks
Shankha Banerjee


On Wed, May 18, 2016 at 1:57 PM, Sameer Kumar <sameer.kumar@ashnik.com> wrote:
>
>
> On Thu, May 19, 2016 at 1:09 AM shankha <shankhabanerjee@gmail.com> wrote:
>>
>> I have the following piece of code:
>>
>>     DROP SCHEMA IF EXISTS s CASCADE;
>>     CREATE SCHEMA s;
>>
>>     CREATE TABLE "s"."t1"
>>     (
>>         "c1" BigSerial PRIMARY KEY,
>>         "c2" BigInt NOT NULL,
>>         "c3" BigInt
>>     )
>>     WITH (OIDS=FALSE);
>>
>>     INSERT INTO s.t1 (c2) VALUES (10);
>>     INSERT INTO s.t1 (c2, c3) VALUES (20, 10);
>>     INSERT INTO s.t1 (c2, c3) VALUES (30, 10);
>>
>>     /* 1. */ SELECT t3.c1 FROM s.t1 as t3 JOIN s.t1 as t2 ON t3.c3 =
>> t2.c2;
>>
>>     /* 2. */ SELECT t1.c1, ARRAY_TO_STRING(ARRAY_AGG((t2.c1)), ',')
>> FROM s.t1 LEFT JOIN  s.t1 as t2
>>     ON t2.c3 = t1.c2 GROUP BY t1.c1;
>>
>>     /* 3. */ SELECT c1, c2,
>>     ARRAY_TO_STRING(ARRAY_AGG((SELECT t3.c1 FROM s.t1 as t3 JOIN s.t1
>> as t2 ON t3.c3 = t2.c2)), ',')
>>     FROM s.t1 t1
>>     GROUP BY c1;
>>     DROP SCHEMA s CASCADE;
>
>
> The query
>
> SELECT t3.c1 FROM s.t1 as t3 JOIN s.t1 as t2 ON t3.c3 = t2.c2 produces
> multiple rows. Since you are calling the aggregate function on the result
> set and not as part of the expression, you are not able to get single row as
> an output.
>
>
>>
>> The output for 1 query:
>>
>>      c1
>>      ----
>>      2
>>      3
>>     (2 rows)
>>
>> 2 Query:
>>
>>      c1 | array_to_string
>>      ----+-----------------
>>        1 | 2,3
>>        2 |
>>        3 |
>>       (3 rows)
>>
>> 3 Query gives me a error:
>>
>>        psql:/tmp/aggregate.sql:24: ERROR:  more than one row returned
>> by a subquery used as an expression
>>
>>
>> The 3 query uses 1 query as inner query. Is there a way to make Query
>> 3 work with inner query as 1 rather than reverting to 2.
>>
>> 3 output should be same as 2.
>>
>> I understand that the error message says query 1 when used as sub
>> query of 3 cannot return more than one row.
>>
>> Pardon my limited knowledge of database.
>>
>>
>> I have tried out:
>>
>>  SELECT c1, c2,
>>     ARRAY_TO_STRING((SELECT ARRAY_AGG(t3.c1) FROM s.t1 as t3 JOIN s.t1
>> as t2 ON t3.c3 = t2.c2), ',')
>>     FROM s.t1 t1
>>     GROUP BY c1;
>>
>
> This would work since the aggregate function has been used on the column.
>
>>
>> Output is :
>>
>>  c1 | c2 | array_to_string
>> ----+----+-----------------
>>   2 | 20 | 2,3
>>   1 | 10 | 2,3
>>   3 | 30 | 2,3
>>
>> Could one of you help me with the correct query.
>>
>>
>
> May you should share some more details of exactly what you are expecting and
> what is the output/corelation you want in the result of the query.
>
>>
>> Thanks
>>
>>
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>
> --
> --
> Best Regards
> Sameer Kumar | DB Solution Architect
> ASHNIK PTE. LTD.
>
> 101 Cecil Street, #11-11 Tong Eng Building, Singapore 069 533
>
> T: +65 6438 3504 | M: +65 8110 0350 | www.ashnik.com