Check out example codes for "sql period overlap". It will help you in understanding the concepts better.

Code Example 1

-- table1          <-----row 1 interval------->
-- table2  <---find this--> <--and this--> <--and this-->

SELECT * 
FROM table1,table2 
WHERE table2.start <= table1.end 
AND (table2.end IS NULL OR table2.end >= table1.start)

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