MySQL 具有 NULL 值的外键

Question

While creating foreign key tables I was expecting foreign key values to be integer type since foreign key references pk values which are integer type. Could you tell me if it is normal behavior and how to get integer type for foreign key values?

def con_engine():
    url = "mysql+pymysql://{user}:{password}@localhost"
    engine = create_engine(url.format(user='root', password=''))
    engine.execute("CREATE DATABASE IF NOT EXISTS data_immo")
    return


def create_table(d_fr, nam, typ, p_k):
    url = "mysql+pymysql://{user}:{password}@localhost/{db}"
    engine = create_engine(url.format(user='root', password='Fer458it', db='data_immo'))
    d_fr.to_sql(name=nam, con=engine, if_exists='append', index=False, dtype=typ)
    engine.execute(f"ALTER TABLE {name} ADD COLUMN {p_k} INT AUTO_INCREMENT PRIMARY KEY")
    return


d = {'df_Bien_Immo': [df_Bien_Immo, 'Bien_Immo', d_b_i, 'Id'],
     'df_Lot': [df_Lot, 'Lot', d_l, 'Id'],
     'df_Cadastre': [df_Cadastre, 'Cadastre', d_c, 'Id'],
     'df_Mutation': [df_Mutation, 'Mutation', d_c, 'Id'],
     'df_Lot_Adr_Voie': [df_Lot_Adr_Voie, 'Lot_Adr_Voie', d_l_a_v, 'Id'],
     'df_Lot_Adr_Commune': [df_Lot_Adr_Commune, 'Lot_Adr_Commune', d_l_a_c, 'Id'],
     'df_Lot_Adr_Dpt': [df_Lot_Adr_Dpt, 'Lot_Adr_Dpt', d_l_a_d, 'Id']}
con_engine()

for key in d:
    df = d[key][0]
    name = d[key][1]
    d_type = d[key][2]
    p_key = d[key][3]
    create_table(df, name, d_type, p_key)
d_fk = {'Bien_Immo': ['fk_Id_Lot', 'Lot', 'Id'],
        'Lot': [['fk_Id_Bien', 'Bien_Immo', 'Id'],
                ['fk_Id_Cad', 'Cadastre', 'Id'],
                ['fk_Id_mut', 'Mutation', 'Id'],
                ['fk_Id_Voie', 'Lot_Adr_Voie', 'Id']],
        'Cadastre': ['fk_Id_Lot', 'Lot', 'Id'],
        'Mutation': ['fk_Id_Lot', 'Lot', 'Id'],
        'Lot_Adr_Voie': [['fk_Id_Lot', 'Lot', 'Id'],
                         ['fk_Id_Com', 'Lot_Adr_Commune', 'Id']],
        'Lot_Adr_Commune': [['fk_Id_Voie', 'Lot_Adr_Voie', 'Id'],
                            ['fk_Id_Dep', 'Lot_Adr_Dpt', 'Id']],
        'Lot_Adr_Dpt': ['fk_Id_Com', 'Lot_Adr_Commune', 'Id']}


def create_fk(name1, name2, name3, name4):
    url = "mysql+pymysql://{user}:{password}@localhost/{db}"
    engine = create_engine(url.format(user='root', password='Fer458it', db='data_immo'))
    engine.execute(f"ALTER TABLE {name1} ADD COLUMN {name2} int, ADD FOREIGN KEY ({name2}) REFERENCES {name3}({name4}) ON DELETE CASCADE")
    return


for key in d_fk.keys():
    if isinstance(d_fk[key][0], list):
        for i in range(len(d_fk[key])):
            [n2, n3, n4] = [d_fk[key][i][0], d_fk[key][i][1], d_fk[key][i][2]]
            print(f'(n2{n2}, n3{n3}, n4{n4})')
            create_fk(key, n2, n3, n4)
    else:
        [n2, n3, n4] = [d_fk[key][0], d_fk[key][1], d_fk[key][2]]
        print(f'(n2{n2}, n3{n3}, n4{n4})')
        create_fk(key, n2, n3, n4)

Many thanks,

Answer 1

I understood that foreign key NULL values problem (since integer type values were expecting) come from that tables were created with 'index=False' option instead of 'True' statement. If no index created in tables, then referencing gets impossible and causes NULL values.