input: Write a python function to query a geospatial database and visualize the data based on a search query.
  assert query_visualize("New York") is not None
  assert query_visualize("San Francisco") is not None
  assert query_visualize("Chicago") is not None
output: def query_visualize(search_query):
      import geopandas as gpd
      import matplotlib.pyplot as plt
      from sqlalchemy import create_engine
  
      # Example connection string to a PostGIS database
      engine = create_engine('postgresql://user:password@localhost/dbname')
  
      # Query the database for the given search term
      query = f"""
      SELECT *
      FROM geospatial_table
      WHERE city_name = '{search_query}'
      """
      
      # Read the query result into a GeoDataFrame
      gdf = gpd.read_postgis(query, engine, geom_col='geom')
  
      if gdf.empty:
          return None
  
      # Plot the GeoDataFrame
      gdf.plot()
      plt.title(f"Geospatial Data for {search_query}")
      plt.show()
  
      return gdf