google-bigquery - Bigquery 如果字段存在

Question

简短:有没有办法在不存在的 BQ 字段中进行查询，并为这些字段接收空值？
我有几乎同样的问题
BigQuery IF field exists THEN但有时我的 API 可以查询没有某些特定字段(历史表)的表，这种方法失败，因为它需要一个包含该字段的表:

SELECT a, b, c, COALESCE(my_field, 0) as my_field
FROM
(SELECT * FROM <somewhere w/o my_field>),
(SELECT * FROM <somewhere with my_field>)

有没有办法做这样的事情:

SELECT IFEXISTS(a, NULL) as the-field
FROM <somewhere w/o my_field>

Answer 1

假设您的表只有 x 和 y 字段!
所以下面的查询将完美地工作

SELECT x, y FROM YourTable

但由于不存在的字段 z，低于 1 将失败

SELECT x, y, z FROM YourTable

解决方法如下

#legacySQL
SELECT x, y, COALESCE(z, 0) as z
FROM 
(SELECT * FROM YourTable),
(SELECT true AS fake, NULL as z)
WHERE fake IS NULL

EDIT: added explicit #legacySQL to not to confuse those who is trying to apply this exact approach to Standard SQL :o)