Select first element of a nested list of lists - aws-cli

Question

I am trying to get the first element in each nested list in a list of lists in JMESpath. Specifically, I am trying variations on:
aws redshift-data get-statement-result \
--id 0de36215-f1db-421f-8704-3242e1a03f21 \
--query 'Records[][0]'
Unfortunately this results in the parent list being firstly flattened, and thus I get just the first element of the first list.
The input data (simplified) is:
{
"Records": [
[{"stringValue": "rdsdb"},{"etc": 1}],
[{"stringValue": "testuser"},{"etc": 100}]
],
"ColumnMetadata": [{"etc": true}],
"TotalNumRows": 2
}
Given that input data, the output I would expect from that:
[
{"stringValue": "rdsdb"},
{"stringValue": "testuser"}
]
How can I get the first element in each nested list in a list of lists?

Answer 1

From what I gather from your comment, you over-simplified your example and you actually after a whole object, and not a single property.
With that requirement, you can achieve it using [*] instead of [], as the first one does not flatten projections, compared to the second one.
So, given the JSON input:
{
"Records": [
[{"stringValue": "rdsdb", "some-property": "foo"},{"etc": 1}],
[{"stringValue": "testuser"},{"etc": 100}]
],
"ColumnMetadata": [{"etc": true}],
"TotalNumRows": 2
}
The query:
Records[*][0]
Yields the expected:
[
{
"stringValue": "rdsdb",
"some-property": "foo"
},
{
"stringValue": "testuser"
}
]
Otherwise, you can filter out all the object in the array of array Records using a filter projection that would look if the key stringValue is not null with [?stringValue]
But because [] is creating a projection, as you raised, you have to stop that projection first, with the pipe operator |, as explained in the linked documentation chapter.
So given:
Records[]|[?stringValue]
This yields the expected:
[
{
"stringValue": "rdsdb"
},
{
"stringValue": "testuser"
}
]