Li Kou achieved the second largest result

To get the second highest , Need to sort , Use order by（ The default is ascending asc, From small to large ）, If you want to reduce the order, use the keyword desc

duplicate removal , If there are multiple identical data , Use keywords distinct duplicate removal

Judge the critical output , If there is no second highest salary , The query should return null, Use ifNull（ Inquire about ,null） Method

names , Use keywords as …

Because it's heavy , And then sort in order , Use limit Method , Query the second largest data , The second highest salary , namely limit 1,1（ Because the default is from 0 Start , So the first one 1 Is the second largest number of queries , the second 1 It means how many pieces of data will be displayed in the future , Only one is needed here ）

select ifNull((
select distinct Salary
from Employee
order by Salary desc limit 1,1),null) as SecondHighestSalary

Key points ifnull and limit

IFNULL and COALESCE difference ?

IFNULL and COALESCE It is to deal with the case that the first value is empty .

ifnull： If it's empty, we'll fill it in

I want to see others coalesce

coalesce(a,b,c), If a Empty value , use b fill , If b Empty value , use c fill ！！！！！！！！

The difference lies in IFNULL Only two parameters are allowed ,COALESCE It can have two or more parameters

---

NULLIF What is it? ?
NULLIF(expression1 ,expression2) If expression1=expression2 Then return to null, otherwise expression1

---

limit

usage

1.limit a( The first four articles )
2.limit a,b（ Subscript 0 At the beginning ）
3.limit 4 offset 10 （ Query the first 11,12,13,14 Four ）
4. To retrieve all record lines from an offset to the end of the recordset , The second parameter can be specified as -1—————————— LIMIT 95 , - 1 // Retrieve record lines 96 - last.

limit() Methods cannot be involved in operations
therefore limit（N+1,1) It's wrong.