Zero foundation self-study SQL course | if function

Hello everyone , I'm Ning Yi .
Today is our first 22 course ：IF function .
If you have learned other programming languages , Should be right IF Functions are familiar . Used according to conditions , Return different values .
Basic grammar ：

IF( Conditional expression ,  value 1,  value 2)

If the conditional expression is True, Return value 1, by False, Return value 2.
The return value can be any value , such as ： The number 、 Text 、 date 、 Null value NULL、 Mathematical expression 、 Functions, etc .
1、 IF function
example ： stay Students In the table , take 1995 Students born in and before are divided into 1 class ,1995 Students born after are divided into 2 class , Finally, it shows Sname,Sage, Class 3 Column .

Sample results ：

SELECT  Sname,  Sage,  IF(    YEAR(Sage) <=1995, "1 class ","2 class "  ) AS " In class "FROM Students;

2、IF Nested function
When the classification is greater than two , Can be IF Function nesting implementation .
example ： stay Students In the table , Student number Sid Less than or equal to 3 Of the students , Belong to 1 class ;Sid by 4 To 6 Of the students , Belong to 2 class ;Sid Greater than or equal to 7 Students of 3 class . Finally, it shows Sname,Sage, Class 3 Column .

SELECT  Sid,  Sname,  IF(Sid<=3,"1 class ",    IF(Sid>=7,'3 class ','2 class ')  ) AS " In class "FROM Students

Nested functions still seem a bit cumbersome , If there are more than two categories , It can be used CASE Keywords replace IF Nested function , In the next class, we will explain in detail .
3、IF+ Aggregate functions
IF Functions are also often used in conjunction with aggregate functions , Let's take a look at an example .
example ： aggregate Teacher Table and Students, Count the number of students that the head teacher brings , Greater than or equal to 5 Display of “5 People and above ”, The number of people is less than 5 Display of “5 People here ”.

Sample results ：

SELECT  t.Tname,  COUNT(*) AS " Number of students ",  IF(COUNT(*)>=5,"5 People and above ","5 People here ") AS " classification "FROM Teachers tJOIN Students sON t.Tid = s.TidGROUP BY t.Tname;

Next time we will talk about CASE function .
Click on Focus on , Update the course and notify the first time ~
author ： Meow Ningyi

Game programming , A game development favorite ~

If the picture is not displayed for a long time , Please use Chrome Kernel browser .