feat(sql-1): solve problem 1 and problem 2 and problem 3

problem1-595-Big-countries.sql

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+Select name,population,area from World
+where area >= 3000000 or population >= 25000000

problem2-177-nthhighest-salary.sql

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+CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
+BEGIN
+  DECLARE M INT;
+  SET M = N-1;
+  RETURN (
+      # Write your MySQL query statement below.
+      select
+       distinct salary
+      from
+        Employee
+      order by salary desc
+      LIMIT M,1
+  );
+END

problem3-196-delete-duplicate-emails.sql

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+delete p1
+from
+person p1
+cross join person p2
+where p1.email = p2.email
+and p1.id > p2.id