ST_BuildArea

ST_BuildArea (A: Geometry)

SELECT ST_BuildArea(
    ST_GeomFromWKT('MULTILINESTRING((0 0, 20 0, 20 20, 0 20, 0 0),(2 2, 18 2, 18 18, 2 18, 2 2))')
) AS geom

+----------------------------------------------------------------------------+
|geom                                                                        |
+----------------------------------------------------------------------------+
|POLYGON((0 0,0 20,20 20,20 0,0 0),(2 2,18 2,18 18,2 18,2 2))                |
+----------------------------------------------------------------------------+