Usage of if conditional statements in SQL

IF expression

IF( expr1 , expr2 , expr3 )

expr1 The value of is TRUE, The return value is expr2
expr1 The value of is FALSE, The return value is expr3

as follows ：

SELECT IF(TRUE,1+1,1+2);
-> 2

SELECT IF(FALSE,1+1,1+2);
-> 3

SELECT IF(STRCMP("111","222")," It's not equal "," equal ");
->  It's not equal 

So this IF What's the use ？ for instance ：
Find a sale price of 50 The book of , If it is java The words of the book , You have to label it as It's sold out
So the corresponding SQL How to write a sentence ？

select *,if(book_name='java',' It's sold out ',' In stock ') as product_status from book where price =50

IFNULL expression

IFNULL( expr1 , expr2 )

stay expr1 The value of the  NULL All cases return expr1, Otherwise return to expr2, as follows ：

SELECT IFNULL(NULL,"11");
-> 11

SELECT IFNULL("00","11");
-> 00

 from  https://blog.csdn.net/zjq_1314520/article/details/62467086