SQL Query to fetch data between starting and ending null - sql

Question

I got a requirement in some project where i need data from a table where i need to select top values that starts after a null value and than again some null values and further data
suppose table name is Data which has a single column named as Company
Company
NULL
NULL
NULL
Google
Microsoft
Oracle
NULL
NULL
Linked In
Twitter
Facebook
NULL
NULL
in simple words i need to write a query that selects {Google, Microsoft and Oracle} as a result and nothing else .... its just a sample data not my actual project
this data can have hundreds of values between starting null and ending null
Please provide your input to write such query
Thanks

Answer 1

Assuming ID is there to keep order... you could do this. This just find the first non-null ID, and then the next null id - 1 as its range.
SELECT *
FROM COMPANY
WHERE ID BETWEEN
( SELECT MIN(ID)
FROM COMPANY
WHERE COMPANY_NAME IS NOT NULL ) AND
( SELECT MIN(ID) - 1
FROM COMPANY
WHERE COMPANY_NAME IS NULL
AND ID > ( SELECT MIN(ID) MIN_ID
FROM COMPANY
WHERE COMPANY_NAME IS NOT NULL ) )
SQL Fiddle Example
Or you could use analytical functions, which would probably be better, assuming you have them in your DBMS.

Answer 2

The easiest way is to use lag():
select d.*
from (select d.*, lag(company) over (order by id) as prev_company
from data d
) d
where prev_company is null and company is not null;
If you don't have lag(), I would use a correlated subquery:
select d.*
from (select d.*,
(select d2.company
from data d2
where d2.id < d.id
order by d2.id desc
limit 1
) as prev_company
from data d
) d
where prev_company is null and company is not null;
Note that in some databases limit might be fetch first row only or select top 1 or some other construct.