PostgreSQL: Sub-select inside insert - sql

Question

I have a table called map_tags:
map_id | map_license | map_desc
And another table (widgets) whose records contains a foreign key reference (1 to 1) to a map_tags record:
widget_id | map_id | widget_name
Given the constraint that all map_licenses are unique (however are not set up as keys on map_tags), then if I have a map_license and a widget_name, I'd like to perform an insert on widgets all inside of the same SQL statement:
INSERT INTO
widgets w
(
map_id,
widget_name
)
VALUES (
(
SELECT
mt.map_id
FROM
map_tags mt
WHERE
// This should work and return a single record because map_license is unique
mt.map_license = '12345'
),
'Bupo'
)
I believe I'm on the right track but know right off the bat that this is incorrect SQL for Postgres. Does anybody know the proper way to achieve such a single query?

Answer 1

Use the INSERT INTO SELECT variant, including whatever constants right into the SELECT statement.
The PostgreSQL INSERT syntax is:
INSERT INTO table [ ( column [, ...] ) ]
{ DEFAULT VALUES | VALUES ( { expression | DEFAULT } [, ...] ) [, ...] | query }
[ RETURNING * | output_expression [ [ AS ] output_name ] [, ...] ]
Take note of the query option at the end of the second line above.
Here is an example for you.
INSERT INTO
widgets
(
map_id,
widget_name
)
SELECT
mt.map_id,
'Bupo'
FROM
map_tags mt
WHERE
mt.map_license = '12345'

Answer 2

INSERT INTO widgets
(
map_id,
widget_name
)
SELECT
mt.map_id, 'Bupo'
FROM
map_tags mt
WHERE
mt.map_license = '12345'

Answer 3

Quick Answer:
You don't have "a single record" you have a "set with 1 record"
If this were javascript: You have an "array with 1 value" not "1 value".
In your example, one record may be returned in the sub-query,
but you are still trying to unpack an "array" of records into separate
actual parameters into a place that takes only 1 parameter.
It took me a few hours to wrap my head around the "why not".
As I was trying to do something very similiar:
Here are my notes:
tb_table01: (no records)
+---+---+---+
| a | b | c | << column names
+---+---+---+
tb_table02:
+---+---+---+
| a | b | c | << column names
+---+---+---+
|'d'|'d'|'d'| << record #1
+---+---+---+
|'e'|'e'|'e'| << record #2
+---+---+---+
|'f'|'f'|'f'| << record #3
+---+---+---+
--This statement will fail:
INSERT into tb_table01
( a, b, c )
VALUES
( 'record_1.a', 'record_1.b', 'record_1.c' ),
( 'record_2.a', 'record_2.b', 'record_2.c' ),
-- This sub query has multiple
-- rows returned. And they are NOT
-- automatically unpacked like in
-- javascript were you can send an
-- array to a variadic function.
(
SELECT a,b,c from tb_table02
)
;
Basically, don't think of "VALUES" as a variadic
function that can unpack an array of records. There is
no argument unpacking here like you would have in a javascript
function. Such as:
function takeValues( ...values ){
values.forEach((v)=>{ console.log( v ) });
};
var records = [ [1,2,3],[4,5,6],[7,8,9] ];
takeValues( records );
//:RESULT:
//: console.log #1 : [1,2,3]
//: console.log #2 : [4,5,7]
//: console.log #3 : [7,8,9]
Back to your SQL question:
The reality of this functionality not existing does not change
just because your sub-selection contains only one result. It is
a "set with one record" not "a single record".