Trying to FOR LOOP a column with ST_CONTAINS in PgAdmin4 - postgresql

Question

I am working with PgAdmin4 to create a View that consists of a large set of geometric data. Part of this data is polylines that exist within polygons. I am attempting to write a code that can loop through all of my polyline data in a given column, and check if it is in a given polygon, and return true/false. So far this is what I have.
DO
$$
BEGIN
FOR i IN (SELECT "geom" FROM "street map"."segment_id")
LOOP
SELECT ST_CONTAINS(
(SELECT "geom" FROM "street map"."cc_districts" WHERE "district number" = 1),
(i)
)
RETURN NEXT i
END LOOP;
END
$$
The error I receive when running this code is as follows:
ERROR: loop variable of loop over rows must be a record or row variable or list of scalar variables
LINE 4: FOR i IN (SELECT "geom" FROM "street map"."segment_id")
^
SQL state: 42601
Character: 18
From what I understand, "i" must refer to a "row variable", and I tried to define that variable with this piece of code:
(SELECT "geom" FROM "street map"."segment_id")
Any ideas to get this going would be very helpful.

Answer 1

A simple join would be much more efficient here
SELECT line.*, polygon.id IS NOT NULL AS is_in_polygon
FROM line
LEFT JOIN polygon
ON ST_Contains(polygon.geometry, line.geometry)
AND polygon.id = 1
Which can be translated as:
Get every field of a line record, and true if the polygon.id exists (is not null), false otherwise (more below). Name this boolean field is_in_polygon.
Do this on every line.
Join (link) each line to the polygon layer. If there is no match, keep the line information and put NULL for every polygon field (this is a left join). If there is a match, keep both line and polygon information.
A match is found if the polygon.geometry contains the line.geometry and if the polygon.id = 1

Answer 2

I have found a way to make this work without doing a JOIN or needing a FOR LOOP. The following code works.
SELECT * FROM "street map"."segment_id"
WHERE
ST_WITHIN(
ST_CENTROID((ST_SetSRID(geom, 4326))),
ST_SetSRID((SELECT geom FROM "street map"."cc_districts" WHERE "district number" = 1),4326))
This lets me do what I was intending by runing the process on all rows in a given column. I swapped from ST_CONTAINS to ST_WITHIN, and I also am now checking if the centroid of the polyline is within the given polygon by using ST_CENTROID. I found that the error goes away by asserting the SRID of the geometry to 4326 using ST_SetSRID. I'm not sure why that works, as my geoms already have an SRID of 4326.
Thanks for all of those who answered