是否有 UDF 可以帮助修剪每个表中的所有字符串空间？答案

【问题标题】：Is there a UDF can help to trim all the string space in each one table?是否有 UDF 可以帮助修剪每个表中的所有字符串空间？
【发布时间】：2021-11-20 01:01:57
【问题描述】：

我是一个 bigquery 用户，这里需要 修剪所有空格 for (data type=string) in table。

通常，每个表中有数百个字段。
如果我一个一个地使用它，它是非常低效的。

能否有更好的解决方案或创建 UDF 来解决？

示例表：

with `project.dataset.tablename` as 
(
select " 1" as number," Male" as Sex,12 as Age,"A " as level union all 
select " 2" as number," Male" as Sex,11 as Age,"A- " as level union all 
select " 3" as number,"Female " as Sex,9 as Age," A " as level union all 
select "4 " as number,"Female" as Sex,13 as Age,"   A " as level union all 
select "5 " as number,"Male " as Sex,10 as Age," B" as level
)

【问题讨论】：

标签： google-bigquery bigquery-udf

【解决方案1】：

如果您需要修剪 BigQuery 表中 STRING 列的所有空格，可以使用TRIM() function。

使用您的示例表：

WITH table_A as 
(
select " 1" as number," Male" as Sex,12 as Age,"A " as level union all 
select " 2" as number," Male" as Sex,11 as Age,"A- " as level union all 
select " 3" as number,"Female " as Sex,9 as Age," A " as level union all 
select "4 " as number,"Female" as Sex,13 as Age,"   A " as level union all 
select "5 " as number,"Male " as Sex,10 as Age," B" as level
)

SELECT TRIM(number), TRIM(Sex), Age, TRIM(level) from table_A

另一方面，基于此StackOverflow post，UDF 函数不能用于创建动态 SQL 语句。

不过，您可以使用以下方法之一：

1- 存储过程

DECLARE i INT64 DEFAULT 1;
DECLARE j INT64 DEFAULT 0;
DECLARE x INT64 DEFAULT 1;
DECLARE z INT64 DEFAULT 0;
DECLARE update_query STRING;
DECLARE string_query STRING;
DECLARE column_name STRING;
DECLARE project_id STRING;
DECLARE dataset_name STRING;
DECLARE table_name STRING;

SET project_id = '<project_id>';
SET dataset_name = '<dataset_name>';
SET table_name = '<table_name>';

SET string_query = 'CREATE OR REPLACE TEMP TABLE temp_tables AS '||
'SELECT table_name, RANK() OVER(ORDER BY table_name) rownum '||
'FROM (SELECT table_name '||
        'FROM '||project_id||'.'||dataset_name||'.INFORMATION_SCHEMA.TABLES '||
        'WHERE table_schema = "'||dataset_name||'"';

IF table_name <> '' THEN
    SET string_query = string_query || ' AND table_name="'||table_name||'"';
END IF;

SET string_query = string_query ||')';

SELECT string_query;
EXECUTE IMMEDIATE string_query;

SET string_query = 'SELECT COUNT(*) FROM temp_tables';
EXECUTE IMMEDIATE string_query INTO j;

      WHILE i<=j DO
            SET table_name = (SELECT table_name FROM temp_tables WHERE rownum = i);
            
            SET string_query = 'CREATE OR REPLACE TEMP TABLE temp_columns AS '||
            'SELECT column_name, RANK() OVER(ORDER BY column_name) rownum '||
            'FROM ('||
                'SELECT column_name '||
                'FROM '||project_id||'.'||dataset_name||'.INFORMATION_SCHEMA.COLUMNS '||
                'WHERE table_name = "'||table_name||'" AND data_type="STRING")';

            EXECUTE IMMEDIATE string_query;

            SET string_query = 'SELECT COUNT(*) FROM temp_columns';
            EXECUTE IMMEDIATE string_query INTO z;

            SET update_query = "";

            WHILE x<=z DO
                SET column_name = (SELECT column_name FROM temp_columns WHERE rownum = x);
                SET update_query = update_query||column_name||"=TRIM("||column_name||")";

                IF x<z THEN
                    SET update_query = update_query || ',';
                END IF;
                SET x=x+1;
            END WHILE;
            SET x = 1;
            SET string_query = 'UPDATE `'||project_id||'.'||dataset_name||'.'||table_name||'` SET '||update_query||' WHERE 1=1';

            EXECUTE IMMEDIATE string_query;
            DROP TABLE temp_columns;
            SET i = i+1;
    END WHILE;
DROP TABLE temp_tables;

2-BigQuery client libraries动态创建句子：

查询表元数据获取列名和列类型
利用这些信息动态创建修剪查询语句

例如与Python library:

from google.cloud import bigquery

client = bigquery.Client()

def get_trim_query(project, dataset, table):
    
    query_job = client.query(
        """
        SELECT column_name, data_type
      FROM {}.{}.INFORMATION_SCHEMA.COLUMNS
      WHERE table_name = '{}'""".format(project,dataset,table)
    )

    results = query_job.result()  # Waits for job to complete.
    
    metadata = []

    for row in results:
        if row.data_type == "STRING":
            metadata.append("TRIM(%s)"%(row.column_name))
        else:
            metadata.append("%s"%(row.column_name))
    
    query = "SELECT %s FROM {}.{}.{}".format(project, dataset, table) % ', '.join(metadata)

    return query

if __name__ == "__main__":
    query = get_trim_query("<PROJECT>","<DATASET>","<ORIGIN_TABLE>")
    print(query)

    destination_table_id = "<PROJECT>.<DATASET>.<DESTINATION_TABLE>"

    job_config = bigquery.QueryJobConfig(destination=destination_table_id)

    sql = query

    query_job = client.query(sql, job_config=job_config)  # Make an API request.
    query_job.result()  # Wait for the job to complete.

    print("Query results loaded to the table {}".format(destination_table_id))

【讨论】：