Difficulty: simple

• 175. Combining two tables

select p.FirstName,p.LastName,a.City,a.State from Person p
left join Address a on
p.PersonId=a.PersonId;

• 176. Second highest salary

select IFNULL 
((select distinct Salary from Employee order by Salary DESC Limit 1,1),NULL)
as SecondHighestSalary;

• 181. Employees in excess of manager's income

select a.Name as Employee from Employee a,Employee b 
where a.ManagerId=b.Id and a.Salary > b.Salary

• 182. Find duplicate email

select Email from Person group by Email having Count(Email) > 1

• 183. Customers who never order

select Name as Customers from Customers where Id not in
(select c.Id from Orders o
left join Customers c
on c.Id=o.CustomerId)

• 196. Delete duplicate email

DELETE from Person WHERE NOT EXISTS (
SELECT c.Id FROM
    (SELECT Min(Id) as Id FROM Person GROUP BY Email)c
     WHERE c.Id=person.Id);

• 197. Rising temperature

SELECT
    weather.Id
FROM
    (
        SELECT
            DATE_ADD(w.RecordDate, INTERVAL 1 DAY) AS RecordDate,
            w.Temperature
        FROM
            weather w
    ) c,
    weather
WHERE
    c.RecordDate = weather.RecordDate
AND c.Temperature < weather.Temperature;

• 595. Big countries

select name,population,area from World where population > 25000000 or area > 3000000

• 596. Lessons for more than five students

SELECT
    class
FROM
    (
        SELECT DISTINCT
            student,
            class
        FROM
            courses
    ) c
GROUP BY
    c.class
HAVING
    count(c.class) >= 5

• 620. Interesting movies

select * from cinema where id%2=1 and description <> "boring" order by rating desc;

• 627. Exchange of wages

UPDATE salary set sex = CASE WHEN (sex='f') THEN 'm' ELSE 'f' END