A combination of two tables
The same applies to the entire inner join Table A and Table B of the additional properties: jion: the connector
An outer connector: left join attribute applies to all of the left and right plus Table A and Table B attributes matching the query conditions, i.e., the left primary table the table. Similarly, right join right table the primary table.
select FirstName,LastName,City,State
from Person left outer join Address
on Person.PersonID = Address.PersonID
Second, the second-highest salary
IFNULL (value1, value2)
. 1, value1 if not empty, return the result value1.
2, if value1 is empty, the result returns value2.
Back Limit x how many data limit x, y + 1 starts from the section X, are y returned.
distinct data de-duplication, then de-duplication of data in descending order, i.e., beginning with the limit 1,1 second, a data fetch, also taken to the second high salary.
select ifnull((
select distinct Salary
from Employee group by Salary desc
limit 1,1),null)
as SecondHighestSalary
Note: ifnull Do not forget the value2, value1 parentheses sql statement too long