Distinguishing which value is obtained from COALESCE by concatenating the value to a different prefix - sql

Question

SELECT COALESCE(grouped_wells.groupforecasting_id,wells.id) as the_id, string_agg(wells.name,', ') as well_name, sum(gas_cd) as gas_cd, date
FROM productions
INNER JOIN completions on completions.id = productions.completion_id
INNER JOIN wellbores on wellbores.id = completions.wellbore_id
INNER JOIN wells on wells.id = wellbores.well_id
INNER JOIN fields on fields.id = wells.field_id
INNER JOIN clusters on clusters.id = fields.cluster_id
LEFT JOIN grouped_wells on grouped_wells.wells_id = wells.id
LEFT JOIN groupforecasting on groupforecasting.id = grouped_wells.groupforecasting_id and groupforecasting.workspace_id = 3
GROUP BY the_id, productions.date
ORDER BY the_id, productions.date
In the above SQL, I am grouping by the_id that results from COALESCE. I want to avoid grouping grouped_wells.groupforecasting_id of equal value to wells.id.
One possible way is to add a prefix depending on which of the two value I got from COALESCE, either add "group_" or "well_".
How to add this conditional prefix to the_id?
If grouped_wells.groupforecasting_id is Null then concatenate "well_" to the result of COALESCE.
Else concatenate "group_" to the result of COALESCE.

Answer 1

You nearly had the solution in your
If grouped_wells.groupforecasting_id is Null then concatenate "well_"
to the result of COALESCE. Else concatenate "group_" to the result of
COALESCE.
In postgres you can use the case when syntax:
case when ... then ...
else ...
end as the_id
completing it in your select (and your comment):
SELECT
case when grouped_wells.groupforecasting_id is NULL then concat('well_', wells.id)
else
concat('group_', grouped_wells.groupforecasting_id)
end as the_id,
string_agg(wells.name,', ') as well_name,
sum(gas_cd) as gas_cd, date
FROM productions
INNER JOIN completions on completions.id = productions.completion_id
INNER JOIN wellbores on wellbores.id = completions.wellbore_id
INNER JOIN wells on wells.id = wellbores.well_id
INNER JOIN fields on fields.id = wells.field_id
INNER JOIN clusters on clusters.id = fields.cluster_id
LEFT JOIN grouped_wells on grouped_wells.wells_id = wells.id
LEFT JOIN groupforecasting on groupforecasting.id = grouped_wells.groupforecasting_id and groupforecasting.workspace_id = 3
GROUP BY the_id, productions.date
ORDER BY the_id, productions.date