Question

maybe someone can help me out with a postgres query.

the table structure looks like this

  nummer   nachname   vorname   cash
 +-------+----------+----------+------+
   2       Bert       Brecht    0,758
   2       Harry      Belafonte 1,568
   3       Elvis      Presley   0,357
   4       Mark       Twain     1,555
   4       Ella       Fitz      0,333
   …

How can I coalesce the fields where “nummer” are the same and sum the cash values?
My output should look like this:

   2       Bert, Brecht      2,326
           Harry, Belafonte 
   3       Elvis, Presley    0,357
   4       Mark, Twain       1,888
           Ella, Fitz   

I think the part to coalesce should work something like this:

 array_to_string(array_agg(nachname|| ', ' ||coalesce(vorname, '')), '<br />') as name,   

Thanks for any help,

tony

Answer 1

SELECT
  nummer, 
  string_agg(nachname||CASE WHEN vorname IS NULL THEN '' ELSE ', '||vorname END, E'\n') AS name,
  sum(cash) AS total_cash
FROM Table1
GROUP BY nummer;

See this SQLFiddle; note that it doesn’t display the newline characters between names, but they’re still there.

The CASE statement is used instead of coalesce so you don’t have a trailing comma on entries with a last name but no first name. If you want a trailing comma, use format('%s, %s',vorname,nachname) instead and avoid all that ugly string concatenation business:

SELECT
  nummer, string_agg(format('%s, %s', nachname, vorname), E'\n'),
  sum(cash) AS total_cash
FROM Table1
GROUP BY nummer;

If string_agg doesn’t work, get a newer PostgreSQL, or mention the version in your questions so it’s clear you’re using an obsolete version. The query is trivially rewritten to use array_to_string and array_agg anyway.

If you’re asking how to sum numbers that’re actually represented as text strings like 1,2345 in the database: don’t do that. Fix your schema. Format numbers on input and output instead, store them as numeric, float8, integer, … whatever the appropriate numeric type for the job is.