根据条件更新现有表列

Question

I am trying to write SQL query for following table,

Employee

id | name   | batch |certificate_name | flag
---------------------------------------
  1| stefan | a     | java            | null
  2| ross   | b     | c++             | null
  3| mad    | a     | php             | null

Query is, for each row check occurrences of certificate and if certificate occurs once in table and batch is 'a' then make flag 1 else do nothing

Result should be:

id | name   | batch |certificate_name | flag
---------------------------------------
  1| stefan | a     |java             | 1
  2| ross   | b     |c++              | null
  3| mad    | a     |php              | 1

what I tried is following,

update employee set flag = 1 
where batch = 'a' 
-- how to write sql query condition for check current row certificate occurrence in table
and ??? 

Answer 1

I would first build a list of unique certificates.

SELECT certificate_name FROM Employee GROUP BY certificate_name HAVING count(*) = 1;

Next I would use that when doing an update.

UPDATE Employee
SET flag = 1
WHERE batch = 'a'
 and certificate_name IN (SELECT certificate_name FROM Employee GROUP BY certificate_name HAVING count(*) = 1);