组合两个表

  • 左连接 
-- 解法1
select p.FirstName ,p.LastName,a.City,a.State from Person p  left join  Address a on (a.personId = p.personId);
-- 解法2
SELECT p.FirstName,p.LastName,a.City,a.State FROM Person p LEFT JOIN Address a USING(PersonId );

select max(Salary) as 'SecondHighestSalary'  FROM Employee where salary < (SELECT MAX(Salary) FROM Employee);
  • limit是mysql的语法select * from table limit m,n其中m是指记录开始的index，从0开始，表示第一条记录n是指从第m+1条开始，取n条。 
select ifnull((SELECT distinct 
    Salary
FROM
    Employee
order by  Salary desc 
limit 1,1),null) AS SecondHighestSalary