How does BigQuery manage a struct field in a SELECT - sql

Question

The following queries a struct from a public data source:
SELECT year FROM `bigquery-public-data.words.eng_gb_1gram` LIMIT 1000
Its schema is:
And the resultset is:
It seems BigQuery automatically translates a struct to all its (leaf) fields when accessed, is that correct? Or how does BigQuery handle directly calling a struct in a select statement?

Answer 1

Two things are going on. You have an array of structs (aka "records").
Each element of the array appears on a separate line in the result set.
Each field in the struct is a separate column.
So, your results are not for a struct but for an array of structs.
You can see what happens for a single struct using:
select year[safe_ordinal(1)]
from . . .
You will get a single row for each row in the data, with the first element of the year array in the row. It will have separate columns, with the names of year.year, year.term_frequency and so on. If you wanted these as "regular" columns, you can use:
select year[ordinal(1)].*
from . . .
Then the columns are year, term_frequency, and so on.

Answer 2

As you might know - RECORD can be NULLABLE - in this case it is a STRUCT and RECORD can be REPEATED - in this case it is an array of record
You can use dot-start notion with the struct to select out all its fields as you do with tables' individual rows with SELECT * FROM tbl or its equivalent SELECT t.* FROM tbl t
So, for example below code
with tbl as (
select struct(1 as a, 2 as b, 3 as c) as col_struct,
[ struct(11 as x, 12 as y, 13 as z),
struct(21, 22, 23),
struct(31, 32, 33)
] as col_array
)
select col_struct.*
from tbl
produces
as if those are the rows of "mini" table called col_struct
Same dot-star notion - does not work for arrays - if you want to output separately elements of array - you need to first to unnest that array. like in below example
with tbl as (
select struct(1 as a, 2 as b, 3 as c) as col_struct,
[ struct(11 as x, 12 as y, 13 as z),
struct(21, 22, 23),
struct(31, 32, 33)
] as col_array
)
select rec
from tbl, unnest(col_array) rec
which outputs
And now, because each row is a struct - you can use dot-star notion
select rec.*
from tbl, unnest(col_array) rec
with output
And, finally - you can combine above as
select col_struct.*, rec.*
from tbl t, t.col_array rec
with output
Note: from tbl t, t.col_array rec is a shortcut for from tbl, unnest(col_array) rec
One more note - if you reference field name that is used in multiple places of your schema - the engine picks most outer matching one. And if by chance this matching one is within the ARRAY - you first need to unnest that array. And if this one is part of STRUCT - you need to make sure you fully qualify the path
For example - with above simplified data
select a from tbl // will not work
select col_struct.a from tbl // will work
select col_array.x from tbl // will not work
select x from tbl, unnest(col_array) // will work
There are many more can be said about subject based on what exactly your use case - but above is some hopefully helpful basics