completed sql1 problems

01-BigCountries.sql

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+-- Problem 1: Big Countries (https://leetcode.com/problems/big-countries/)
+select name, population, area 
+from world
+where area >= 3000000 or population >=25000000

02-NthHighestSalary.sql

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+-- Problem 2: Nth Highest Salary (https://leetcode.com/problems/nth-highest-salary/)
+
+CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
+BEGIN
+RETURN (
+    # Write your MySQL query statement below.
+    with cte as (
+        select salary, dense_rank()
+        over (order by salary desc) as rnk
+        from employee
+    )
+    select distinct ifnull(salary, null)
+    from cte
+    where rnk=N
+  );
+END

03-DeleteDuplicateEmails.sql

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+-- Problem 3: Delete Duplicate Emails (https://leetcode.com/problems/delete-duplicate-emails/)
+
+delete p1 from person p1
+cross join person p2
+where p1.email = p2.email and p1.id > p2.id