gh-79097: Add support for aggregate window functions in sqlite3 (GH-20903)

Doc/includes/sqlite3/sumintwindow.py

@@ -0,0 +1,46 @@
+# Example taken from https://www.sqlite.org/windowfunctions.html#udfwinfunc
+import sqlite3
+
+
+class WindowSumInt:
+    def __init__(self):
+        self.count = 0
+
+    def step(self, value):
+        """Adds a row to the current window."""
+        self.count += value
+
+    def value(self):
+        """Returns the current value of the aggregate."""
+        return self.count
+
+    def inverse(self, value):
+        """Removes a row from the current window."""
+        self.count -= value
+
+    def finalize(self):
+        """Returns the final value of the aggregate.
+
+        Any clean-up actions should be placed here.
+        """
+        return self.count
+
+
+con = sqlite3.connect(":memory:")
+cur = con.execute("create table test(x, y)")
+values = [
+    ("a", 4),
+    ("b", 5),
+    ("c", 3),
+    ("d", 8),
+    ("e", 1),
+]
+cur.executemany("insert into test values(?, ?)", values)
+con.create_window_function("sumint", 1, WindowSumInt)
+cur.execute("""
+    select x, sumint(y) over (
+        order by x rows between 1 preceding and 1 following
+    ) as sum_y
+    from test order by x
+""")
+print(cur.fetchall())