Changing NULL without ISNULL etc

Hello,

I've been set a 'challenge' query. Basically if a certain column has NULL values, I need to replace those NULL values with another value (obtained from a subquery). So I thought, hey that's easy, I'll use ISNULL/COALESCE however I've been told I can't and instead I can only use JOINS/UNIONS and IS NULL.

Unfortunately I don't know where to start. I was wondering if you could give me some pointers as to how it would be done generally using JOINS/UNIONS and IS NULL.

Thanks in advance.

You are somewhat vague regarding where to source the "another value (obtained from a subquery)" and this detail is an imporant part of creating the correct syntax to get it.

Based on the brief explanation that you have given, I can suggest you start with either;

UPDATE
FirstTable SET Column01 =--(subquery)
WHERE
Column01 IS NULL


Or you can try...

UPDATE
FirstTable SET FirstTable.Column01 = SecondTable.ColumnNewValue FROM FirstTable, SecondTable
WHERE
FirstTable.UniqueKeyField = SecondTable.UniqueKeyField AND FirstTable.Column01 IS NULL

Just as addition;
Your initial post did not specify that your intended query was a DAL (select) type, which is why I proposed an UPDATE query.
To achieve this as a SELECT, I would suggest something like;

SELECT Column01 from FirstTable WHERE Column01 IS NOT NULL
UNION ALL
SELECT SecondTable.ColumnNewValue FROM SecondTable
INNER JOIN FirstTable ON FirstTable.UniqueKeyField = SecondTable.UniqueKeyField 
WHERE FirstTable.Column01 IS NULL