Concat in function - sql-server

Question

I've an issue when I try to create a function from a working concat select
Here the function
CREATE FUNCTION dbo.afEvrGetConsequencesConcat
(#id_evr int,#type_evr int)
RETURNS varchar(max)
AS
BEGIN
declare #result varchar(max)
set #result= ''
IF #type_evr = 0
BEGIN
SELECT #result = #result + (case when (CEV.ID_CON is null) then CEV.NOM_CEV else CON.NOM_CON end) + ' ; '
from T_CONSEQUENCE_EVRP CEV
left join T_CONSEQUENCE CON on CEV.id_con=CON.id_con
WHERE CEV.id_evr = #id_evr
END
IF #type_evr = 1
BEGIN
SELECT DISTINCT #result = #result + isnull(NOM_PHR,'') + ' ; '
FROM T_EVRP EVR
INNER JOIN T_EVR_CHIM CHIM on EVR.ID_EVR=CHIM.ID_EVR
INNER JOIN T_PRODUIT PROD on CHIM.ID_PROD=PROD.ID_PROD
INNER JOIN T_PRODUIT_PHRASE_R PROD_PHR on PROD.ID_PROD=PROD_PHR.ID_PROD
INNER JOIN T_PHRASE_R PHR on PROD_PHR.ID_PHR=PHR.ID_PHR
WHERE TYPE_EVR=1 AND EVR.id_evr = #id_evr AND PHR.flag_consequence = 1
UNION
SELECT DISTINCT #result = #result + isnull(nom_cons_txt,'') +' ; '
FROM T_EVRP EVR
INNER JOIN T_EVR_CHIM CHIM on EVR.ID_EVR=CHIM.ID_EVR
INNER JOIN T_PRODUIT PROD on CHIM.ID_PROD=PROD.ID_PROD
INNER JOIN t_consequence_txt PROD_CONS on PROD.ID_PROD=PROD_CONS.ID_PROD
WHERE TYPE_EVR=1 AND EVR.id_evr = #id_evr AND PROD.transverse_prod = 1
END
RETURN #result
END
GO
And the message error is 'Msg 444, Niveau 16, État 2, Procédure afEvrGetConsequencesConcat, Ligne 23
Select statements included within a function cannot return data to a client.'
The SELECT above works well outside a function. Each time, I got 1 varchar in #result, that match with the SELECT concat. And this similary function works well too :
CREATE FUNCTION dbo.afEvrGetControleConcat
(#id_evr int,#type_evr int,#type_ctrl int)
RETURNS varchar(8000)
AS
BEGIN
declare #result varchar(8000)
set #result= ''
-- 0 tous
-- 1 date + comm
IF #type_ctrl = 0
BEGIN
SELECT #result = #result + ( CONVERT(VARCHAR(10),date_ctr, 103) + ' - ' + CONVERT(VARCHAR(30),isnull(valeur_mesure,'')) + ' - ' + isnull(unite_mesure,'') + ' - ' + com_ctr ) + ' ;'
FROM t_controle_exposition
WHERE id_evr = #id_evr
END
IF #type_ctrl = 1
BEGIN
SELECT #result = #result + ( CONVERT(VARCHAR(10),date_ctr, 103) + ' - ' + com_ctr ) + ' ;'
FROM t_controle_exposition
WHERE id_evr = #id_evr
END
RETURN #result
END
GO
Then what is the problem ? By logic, I want return a varchar, not a table or any funky data. So I don't unterstand this error message.
Thank for your help.
EDIT: Add a temporary table and insert inside the result of UNION and concat next works well :)
CREATE FUNCTION dbo.afEvrGetConsequencesConcat
(#id_evr int,#type_evr int)
RETURNS varchar(max)
AS
BEGIN
declare #result varchar(max)
set #result= ''
declare #t as table (v varchar(max))
IF #type_evr = 0
BEGIN
SELECT #result = #result + (case when (CEV.ID_CON is null) then CEV.NOM_CEV else CON.NOM_CON end) + ' ; '
from T_CONSEQUENCE_EVRP CEV
left join T_CONSEQUENCE CON on CEV.id_con=CON.id_con
WHERE CEV.id_evr = #id_evr
END
IF #type_evr = 1
BEGIN
insert into #t
SELECT DISTINCT isnull(NOM_PHR,'') + ' ; '
FROM T_EVRP EVR
INNER JOIN T_EVR_CHIM CHIM on EVR.ID_EVR=CHIM.ID_EVR
INNER JOIN T_PRODUIT PROD on CHIM.ID_PROD=PROD.ID_PROD
INNER JOIN T_PRODUIT_PHRASE_R PROD_PHR on PROD.ID_PROD=PROD_PHR.ID_PROD
INNER JOIN T_PHRASE_R PHR on PROD_PHR.ID_PHR=PHR.ID_PHR
WHERE TYPE_EVR=1 AND EVR.id_evr = #id_evr AND PHR.flag_consequence = 1
UNION
SELECT DISTINCT isnull(nom_cons_txt,'') +' ; '
FROM T_EVRP EVR
INNER JOIN T_EVR_CHIM CHIM on EVR.ID_EVR=CHIM.ID_EVR
INNER JOIN T_PRODUIT PROD on CHIM.ID_PROD=PROD.ID_PROD
INNER JOIN t_consequence_txt PROD_CONS on PROD.ID_PROD=PROD_CONS.ID_PROD
WHERE TYPE_EVR=1 AND EVR.id_evr = #id_evr AND PROD.transverse_prod = 1
SELECT #result = #result + v FROM #t
END
RETURN #result
END
GO

Answer 1

I think this is to do with the UNION. Try running:
DECLARE #result varchar(max)
SELECT #result = 'test'
UNION
SELECT #result = 'another'
You will get:
Msg 10734, Level 15, State 1, Line 5
Variable assignment is not allowed in a statement containing a top level UNION, INTERSECT or EXCEPT operator.
If you must do the UNION, you're going to have to return a table, otherwise come up with some other way without the UNION. This could explain why the second function works, as it doesn't have a UNION