What is a "path_expression" in BigQuery

Question

BigQuery describes a path_expression in the Syntax page as follows:

A path expression describes how to navigate to an object in a graph of objects and generally follows this structure...

Examples:

foo.bar
foo.bar/25
foo/bar:25
foo/bar/25-31
/foo/bar
/25/foo/bar

What are some actual examples of a Path Expression with a valid table, for example in a CTE? My thinking was that a path expression would be something that explicitly qualifies a field or struct sub-field, such as:

myTable.myField.mySubfield

But from the above syntax, which allows for:

/:-

I'm not exactly sure what it is or how it would be used. Could someone show a real-world example of how a path expression would be used?

Answer 1

Path expressions describe where to search for (or store) data. Consider this example from the docs:

#legacySQL
SELECT
  weight_pounds, state, year, gestation_weeks
FROM
  [bigquery-public-data:samples.natality]
ORDER BY
  weight_pounds DESC
LIMIT
  10;

In this example, the portion [bigquery-public-data:samples.natality] is the path_expression. It is saying to look at the natality table in the samples database in the bigquery-public-data project.

But from the above syntax, which allows for: /:-

This would actually not be allowed, as a path_expression.

This example would be parsed as:

/:-

{first_part}/{subsequent_part}:{subsequent_part}

{{ unquoted_identifier | quoted_identifier }} / 
{{ unquoted_identifier | quoted_identifier | number }} : 
{{ unquoted_identifier | quoted_identifier | number }}

An unquoted_identifier must at least begin with a letter or an underscore. A quoted_identifier can contain any character, but cannot be empty. The empty string therefore cannot be considered an unquoted_identifier, quoted_identifier, or number, so this expression is invalid (and is invalid in 3 positions).

A possible minimal path_expression could be something like:

a:b.c

meaning look in the c table in the b database in the a project.

Answer 2

I can be wrong, but I think you are confusing Path Expression (as it is defined in referenced documentation) with something like (for example) JSONPath.
In my mind, the the former is just terminology that introduced in order to have consistent reference within the documentation, while later (obviously) is query language for JSON, similar to XPath for XML, etc.

So, if you would asked for something like JSONPath use - I bet you would get tones of usage examples, but asking about Path Expression use has not much chances to gather answers even having hefty bounty as it is ONLY used in that documentation.