Correlated Subquery with Exists

I was updating a query the other day that was screening unwanted duplicates by using a Group By statement on all fields. The goal of the query is to return all records if the search parameter is null, otherwise filter the records by the search parameter. After a little analysis, I realized the duplicates were coming from a join on a Child table where the child table fields were only used for the where clause. The solution I came up with was to use a correlated subquery with Exists in the where clause like:

WHERE
(
(@ChildField IS NULL)
OR EXISTS
(
Select
1
from
ChildTable CT
where
CT.ID = TargetTable.ID
and CT.ChildField like @ChildField
)
)

Optional Parameter Query

I've been working on a SQL Query with optional parameters that returns all records if no parameter(s) are sent, or narrows the query by the parameter(s) if sent.

The key line is in the where clause and looks like:

WHERE COALESCE(MT.Name, '') like COALESCE(@ProductName, MT.Name, '')

Which has the effect : If @ProductName not null then match MT.Name to itself(return all records) and if MT.Name is null match '' to itself ( because Null = Null does not return true in sql)