Postgresql - Update CTE result from CTE Output? - sql-server

Question

I need to update CTE o/p one of the column value (top 1 record) based on the latest timestamp & then return.
Query
WITH cte AS (
select
dt_zone.zone_name,
dt_material_status.mtstatus_name,
dt_historicalzone.visit_time_in
FROM ((public.dt_historicalzone
INNER JOIN dt_material_status
ON dt_historicalzone.mtstatus_id = dt_material_status.mtstatus_id)
INNER JOIN dt_zone ON dt_historicalzone.zone_id = dt_zone.zone_id)
WHERE material_id = 'ELS46885'
ORDER BY dt_historicalzone.zone_id DESC)
UPDATE cte SET cte.mtstatus_name = true WHERE SELECT * FROM cte LIMIT 1;
SELECT * FROM cte

Answer 1

You may try using an update join, with Postgres' syntax, including a CTE for the limit portion of the query:
WITH cte AS (
SELECT dh.mtstatus_id
FROM dt_historicalzone dh
INNER JOIN dt_zone dz
ON dh.zone_id = dz.zone_id
ORDER BY zone_id DESC
LIMIT 1
)
UPDATE dt_material_status d
SET mtstatus_name = true
FROM cte t
WHERE d.mtstatus_id = t.mtstatus_id AND
d.material_id = 'ELS46885';

Answer 2

when you update a CTE, the background table is getting updated. You can have only one statement below CTE. Post that CTE loses its scope. You can only go for UPDATE statement, post CTE. I have modified the CTE and updated the top 1 row.
The above statement is applicable for SQL Server. In Postgres, the CTE cannot be target of UPDATE statements. See the below error in Postgres.
Query Error: error: relation "cte" cannot be the target of a modifying
statement
WITH cte AS (
select top 1
dt_zone.zone_name,
dt_material_status.mtstatus_name,
dt_historicalzone.visit_time_in
FROM public.dt_historicalzone
INNER JOIN dt_material_status
ON dt_historicalzone.mtstatus_id = dt_material_status.mtstatus_id
INNER JOIN dt_zone ON dt_historicalzone.zone_id = dt_zone.zone_id
WHERE material_id = 'ELS46885'
ORDER BY dt_historicalzone.zone_id DESC)
UPDATE cte
SET mtstatus_name = true
I have tried with sample data for a CTE update. Below works fine in SQL Server.
create table #test(a int)
create table #test2(a int, b int)
insert into #test values (1)
insert into #test2 values (1,1)
;WITH CTE as
(
select top 1 t.a, t2.b
FROM #test as t
join #test2 as t2
on t.a = t2.a
order by t.a desc
)
update cte set b = 0
select * from #test2