I thought you couldn't filter a normalized table in agate, but this example works

normalize_working.py

import agate

column_names = ['letter', 'number', 'state', 'city']
column_types = [agate.Text(), agate.Number(), agate.Text(), agate.Text()]

rows = [
    ('a', 1, 'TX', 'Austin'),
    ('b', 2, 'NY', 'Buffalo'),
    ('c', None, 'CA', 'Los Angeles')
]

# making tables
table = agate.Table(rows, column_names, column_types)

print('Normal table print:\n')
table.print_table()

print('\nFiltered table print: \n')
table.where(lambda row: row['state'] == "TX").print_table()

normalized = table.normalize(['state', 'city'], ['letter', 'number'])

print('\nnormalized table: \n')
normalized.print_table()

print('\nfiltered normalized table:\n')
normalized.where(lambda row: row['state'] == 'TX').print_table()