How to find out if a row with a specific primary key exists in a table in SQL
I actually tried to look at this link: https://www.w3schools.com/sql/sql_join.asp 
I describes how to INNER JOIN. It seems that I got the correct result using the below code.

Assuming that testTable is the one with 100 Million Rows
Assuming that testTable2 is the one with 5000 Rows

I could find the exact matches for FeatureNbr and DateTime as a result.
Please tell if I do miss anything there or if I am thinking wrong so I can edit the post.
SELECT testTable.DateTime, testTable.FeatureNbr
FROM testTable
INNER JOIN testTable2 ON testTable.FeatureNbr=testTable2.FeatureNbr AND 
                         testTable.DateTime=testTable2.DateTime;