diff --git a/dataframely/functional.py b/dataframely/functional.py
index 475bb67..89931ee 100644
--- a/dataframely/functional.py
+++ b/dataframely/functional.py
@@ -23,22 +23,43 @@
 
 # --------------------------------- RELATIONSHIP 1:1 --------------------------------- #
 
+LEN_COLUMN = "__len__"
+
 
 def filter_relationship_one_to_one(
     lhs: LazyFrame[S] | pl.LazyFrame,
     rhs: LazyFrame[T] | pl.LazyFrame,
     /,
     on: str | list[str],
+    *,
+    keep_only_unique: bool = False,
 ) -> pl.LazyFrame:
     """Express a 1:1 mapping between data frames for a collection filter.
 
     Args:
         lhs: The first data frame in the 1:1 mapping.
         rhs: The second data frame in the 1:1 mapping.
-        on: The columns to join the data frames on. If not provided, the join columns
-            are inferred from the mutual primary keys of the provided data frames.
+        on: The columns to join the data frames on.
+        keep_only_unique: If `True`, drop non-unique rows from both data frames.
+            This is useful when the join columns do not already uniquely identify rows.
     """
-    return lhs.join(rhs, on=on)
+    if keep_only_unique:
+        return (
+            lhs.group_by(on)
+            .len(LEN_COLUMN)
+            .filter(pl.col(LEN_COLUMN) == 1)
+            .drop(LEN_COLUMN)
+            .join(
+                rhs.group_by(on)
+                .len(LEN_COLUMN)
+                .filter(pl.col(LEN_COLUMN) == 1)
+                .drop(LEN_COLUMN),
+                on=on,
+                how="inner",
+            )
+        )
+    else:
+        return lhs.join(rhs, on=on)
 
 
 # ------------------------------- RELATIONSHIP 1:{1,N} ------------------------------- #
@@ -55,8 +76,7 @@ def filter_relationship_one_to_at_least_one(
     Args:
         lhs: The data frame with exactly one occurrence for a set of key columns.
         rhs: The data frame with at least one occurrence for a set of key columns.
-        on: The columns to join the data frames on. If not provided, the join columns
-            are inferred from the joint primary keys of the provided data frames.
+        on: The columns to join the data frames on.
     """
     return lhs.join(rhs.unique(on), on=on)
 
diff --git a/tests/functional/test_relationships.py b/tests/functional/test_relationships.py
index b650e11..b72e250 100644
--- a/tests/functional/test_relationships.py
+++ b/tests/functional/test_relationships.py
@@ -29,7 +29,7 @@ class EmployeeSchema(dy.Schema):
 
 @pytest.fixture()
 def departments() -> dy.LazyFrame[DepartmentSchema]:
-    return DepartmentSchema.cast(pl.LazyFrame({"department_id": [1, 2]}))
+    return DepartmentSchema.cast(pl.LazyFrame({"department_id": [1, 2, 3]}))
 
 
 @pytest.fixture()
@@ -44,9 +44,9 @@ def employees() -> dy.LazyFrame[EmployeeSchema]:
     return EmployeeSchema.cast(
         pl.LazyFrame(
             {
-                "department_id": [2, 2, 2],
-                "employee_number": [101, 102, 103],
-                "name": ["Huey", "Dewey", "Louie"],
+                "department_id": [2, 2, 2, 3],
+                "employee_number": [101, 102, 103, 104],
+                "name": ["Huey", "Dewey", "Louie", "Daisy"],
             }
         )
     )
@@ -67,6 +67,19 @@ def test_one_to_one(
     assert actual.select("department_id").collect().to_series().to_list() == [1]
 
 
+def test_one_to_one_keep_only_unique(
+    departments: dy.LazyFrame[DepartmentSchema],
+    employees: dy.LazyFrame[EmployeeSchema],
+) -> None:
+    actual = dy.filter_relationship_one_to_one(
+        departments,
+        employees,
+        on="department_id",
+        keep_only_unique=True,
+    )
+    assert actual.select("department_id").collect().to_series().to_list() == [3]
+
+
 def test_one_to_at_least_one(
     departments: dy.LazyFrame[DepartmentSchema],
     employees: dy.LazyFrame[EmployeeSchema],
@@ -74,4 +87,4 @@ def test_one_to_at_least_one(
     actual = dy.filter_relationship_one_to_at_least_one(
         departments, employees, on="department_id"
     )
-    assert actual.select("department_id").collect().to_series().to_list() == [2]
+    assert actual.select("department_id").collect().to_series().to_list() == [2, 3]