Select Parents if name matches on Parent or Child - sql

Question

I want to create an SQL Query that returns a list of Parent rows, if a given name matches either the name of a Parent or the name of a Child. Thus, if the name matches a Parent, return that Parent. If the name matches a Child, return the Parent of that Child. No Parent row should appear more than once in the result.
Consider this example (Run the snippet for some nice HTML tables):
table, th, td {
border: 1px solid black;
border-collapse: collapse;
padding: 3px;
}
<h2>Parent table</h2>
<table>
<tr>
<th>id</th>
<th>name</th>
</tr>
<tr>
<td>1</td>
<td>Karl</td>
</tr>
<tr>
<td>2</td>
<td>Robert</td>
</tr>
</table>
<h2>Child table</h2>
<table>
<tr>
<th>id</th>
<th>parent</th>
<th>name</th>
</tr>
<tr>
<td>1</td>
<td>2</td>
<td>Samuel</td>
</tr>
<tr>
<td>2</td>
<td>1</td>
<td>Karl</td>
</tr>
<tr>
<td>3</td>
<td>1</td>
<td>Robert</td>
</tr>
</table>
<h2>Result for Karl</h2>
<table>
<tr>
<th>id</th>
<th>name</th>
</tr>
<tr>
<td>1</td>
<td>Karl</td>
</tr>
</table>
<h2>Result for Robert</h2>
<table>
<tr>
<th>id</th>
<th>name</th>
</tr>
<tr>
<td>1</td>
<td>Karl</td>
</tr>
<tr>
<td>2</td>
<td>Robert</td>
</tr>
</table>
<h2>Result for Samuel</h2>
<table>
<tr>
<th>id</th>
<th>name</th>
</tr>
<tr>
<td>2</td>
<td>Robert</td>
</tr>
</table>
This is what I've tried so far:
SELECT Parent.*
FROM Parent,
Child
WHERE Parent.name = :name_parameter
OR Child.name = :name_parameter AND Child.parent = Parent.id
It doesn't work though, the result is always empty.

Answer 1

Try this:
select distinct Parent.*
from Parent
left join Child on Child.parent = Parent.id
where Parent.name = :name_parameter
or Child.name = :name_parameter
distinct removes duplicates.
left still returns parents who don’t have children.