Multiple records update validation with the entire data set - oracle

Question

Currently, I have an endpoint that allows the update of multiple records. Before saving the changes to the database I need to validate the data that is being sent from the front end.
I am having an issue with some kinds of validations that require checks against all the other records in the database table. (Ex: ate intervals overlaps/gaps, unique pairs checks).
In these cases when I try to do the validations I have two sets of data
The data sent from the front end that are stored in memory/variable.
The data on the database.
For the validations to be run correctly I need a way to merge the data in memory(the updated records) with the data in a database(original records + other data that is not currently being updated).
Is there a good way of doing this that does not require loading everything on the memory and merging both datasets there?
Another idea I am thinking of is to open a database transaction set the new data to the database and then when executing the gaps/overlap check queries use dirty read. I don't know if this is a good approach though.
Extra notes:
I am using Oracle as a database and Dapper to communicate with it.
The tables that need validation usually hold millions of records.
The same issue is for the create endpoint.
Another example
I am trying to create entities. The create endpoint is called and it has these data on the body (date format dd/mm/yyy):
StartDate
EndDate
01/01/2022
05/01/2022
10/01/2022
11/01/2022
12/01/2022
15/01/2022
In database I have these records saved:
Id
StartDate
EndDate
1
06/01/2022
09/01/2022
2
16/01/2022
20/01/2022
I need to check if there are any gaps between the dates. If there are I need to send a warning to the user(data in the database can be invalid - the application has old data and I can't do anything about that at the moment).
The way I check for this right now is like by using the SQL below
WITH CTE_INNERDATA AS(
SELECT s.STARTDATE, s.ENDDATE
FROM mytable s
WHERE FK = somefkvalue
UNION ALL
SELECT :StartDt AS STARTDATE, :EndDt AS ENDDATE FROM DUAL -- this row contains the data from one of the rows that came form the front-end
),
CTE_DATA AS (
SELECT ctid.STARTDATE, ctid.ENDDATE, LAG(ctid.ENDDATE, 1) OVER (ORDER BY ctid.STARTDATE) AS PREV_ENDDATE FROM CTE_INNERDATA ctid
)
SELECT COUNT(1) FROM cte_data ctd
WHERE PREV_ENDDATE IS NOT NULL
AND PREV_ENDDATE < STARTDATE
Using this SQL query when validating the third row (12/01/2022 - 15/01/2022) there will be a gap between dates 09/01/2022 and 12/01/2022.
This issue would be fixed if instead of using union with a single row, to use it with all the rows send from the front-end but I can't figure out a way to do something like that.
#Update
I iterate through the records the frontend sent and call this method to check for gaps.
private async Task ValidateIntervalGaps(int someFkValue, DateTime startdate, DateTime endDate)
{
var connection = _connectionProvider.GetOpenedConnection();
var gapsCount = await connection.QueryFirstAsync<int>(#"<<Query from above>>",
new { StartDt = startdate, EndDt = endDate, somefkvalue= someFkValue });
if (gapsCount > 0)
{
// Add warning message here
}
}