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 t
JOIN Students s
ON t.Tid = s.Tid
GROUP BY t.Tname;

Next time we will talk about CASE function .

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