T-SQL Foreach row if column has value 0 - sql-server

Question

I'm new in working with SQL queries and I'm trying to check if for each parent from the Parents table I have the age for all children from Children table. For example:
Parents
Name Id
John Smith 7
Children
Name Age ParentId
Sasha Smith 10 7
Johnny Smith 0 7
This is what I have so far:
create function fnCheckChildren(
#parentId int
)
returns int
begin
declare #allOk int
if(select count(1) from Children where ParentId=#parentId ) = 0
BEGIN
SET #allOk =1 --default all are ok - parent has no children
END
else
BEGIN
--here is my missing part
END
return #allOk
end
go
If I'll call the function for John Smith (#parentId = 7) #allOk should have the value 0 because one of his children (Johnny) has Age 0. If all had Age > 0 then #allOk should have 1.
Can anyone help me with it? As I understand, T-SQL doesn't support For Each...Next, and so on, so what alternatives do I have?

Answer 1

Here is a relative simple and efficient method:
select p.*,
(case when exists (select 1 from children c where c.parentid = p.id and c.age = 0
then 1 else 0
end) as flag
from p;
You can make this a function by simply doing:
create function fnCheckChildren(
#parentId int
)
returns int
begin
if (exists (select 1 from children where parentid = #parentid and age = 0)
)
begin
return(0);
end;
return(1);
end;

Answer 2

There is no need for CURSOR at all:
SELECT *
FROM Parents p
OUTER APPLY (SELECT CASE WHEN
COUNT(*) = COUNT(CASE WHEN c.Age > 0 THEN 1 END) THEN 1
ELSE 0
END AS res
FROM Children c
WHERE p.id = c.parent_id
) sub
-- WHERE ....;
DBFiddle Demo
As I understand, T-SQL doesn't support For Each.
This is not true, For-Each is kind of correlated subquery for instance: using inline syntax like in Gordon's answer or OUTER APPLY.

Answer 3

You can try the following query
CREATE FUNCTION fnCheckChildren(
#parentId INT
)
RETURNS INT
BEGIN
DECLARE #allOk INT
SET #allOk = CASE WHEN (select count(*) from Children where ParentId=#parentId AND Age > 0) > 1 THEN 1
ELSE 0 END
RETURN #allOk
END
Thanks

Answer 4

You could also have a case of no children at all. This will indicate that.
declare #P table (id int identity primary key, name varchar(20));
insert into #P (name) values ('John Smith'), ('Pat Jones'), ('Will Smith');
select * from #P;
declare #C table (id int identity primary key, parID int, name varchar(20), age int);
insert into #C (parID, name, age) values (1, 'jim', 12), (1, 'sally', 0), (2, 'bruce', 10);
select * from #C;
select p.id, P.name
, count(c.id) as [child count]
, case when MIN(c.age) <= 0 then 'childen with zero' else 'no children with zero' end as status
from #P p
left join #C c
on p.id = c.parID
group by p.id, p.name;