Question

以下缩短代码请求表并输出表模式：

# pip install google-cloud-bigquery
from google.cloud import bigquery
client = bigquery.Client()
dataset_ref = client.dataset("chicago_crime", project="bigquery-public-data")
dataset = client.get_dataset(dataset_ref)
table_ref = dataset_ref.table("crime")
table = client.get_table(table_ref)
table.schema[:4]

输出：

[SchemaField('unique_key', 'INTEGER', 'REQUIRED', 'Unique identifier for the record.', ()),
 SchemaField('case_number', 'STRING', 'NULLABLE', 'The Chicago Police Department RD Number (Records Division Number), which is unique to the incident.', ()),
 SchemaField('date', 'TIMESTAMP', 'NULLABLE', 'Date when the incident occurred. this is sometimes a best estimate.', ()),
 SchemaField('block', 'STRING', 'NULLABLE', 'The partially redacted address where the incident occurred, placing it on the same block as the actual address.', ())

列出字段（1,3）的代码如下：

from operator import itemgetter
fields_list=itemgetter(1,3)(table.schema)
client.list_rows(table, selected_fields=fields_list, max_results=5).to_dataframe()

输出：

    case_number block
0   JC299491    114XX S CHAMPLAIN AVE
1   JC273204    053XX N LOWELL AVE

如何明确指出字段名称？

fields_list=['case_number', 'block']

Answer 1

您可以创建架构字段名称与其各自的SchemaField对象的反向映射。

类似的东西：

schema_fields = dict((s.name.lower(), s) for s in table.schema)

有了这个，您可以用它们的名字来挑选字段：

fields_list = ['case_number', 'block']
selected_fields = map(lambda n: schema_fields[n], fields_list)
client.list_rows(table, selected_fields=selected_fields, max_results=5).to_dataframe()

SQL和BigQuery字段选择

1 个答案: