MySQL-What's the difference between these two sql?CASE WHEN in MAX(),one uses null,one uses a character

Question

Data source as follows:

name    course
---------------
Marry   UNIX
Bob     Java
Bob     SQL
Cherry  SQL
John    SQL
John    UNIX
Eric    SQL

this sql is correct:

 SELECT  name,
    max(CASE WHEN course = 'SQL' THEN '○' ELSE null END) AS "SQL",
    max(CASE WHEN course = 'UNIX' THEN '○' ELSE null END) AS "UNIX",
    max(CASE WHEN course = 'Java' THEN '○' ELSE null END) AS "Java"
 FROM Courses
 GROUP BY name;

but this is wrong:

 SELECT  name,
    max(CASE WHEN course = 'SQL' THEN '○' ELSE 'x' END) AS "SQL",
    max(CASE WHEN course = 'UNIX' THEN '○' ELSE 'x' END) AS "UNIX",
    max(CASE WHEN course = 'Java' THEN '○' ELSE 'x' END) AS "Java"
 FROM Courses
 GROUP BY name;

I ran the second one in MySQL,and the result is wrong

    name    SQL   UNIX   JAVA
    Bob     x      x      x
    Cherry  ○      x      x
    Eric    ○      x      x
    John    x      x      x
    Marry   x      ○      x

so what's the problem between these two sql? MAX() ignore null?

Answer 1

MAX() ignores null values, and picks the max value otherwise seen. Since 'x' is larger than 'o' , if both values are present, the result is 'x' .

Eg for Bob, since there are 2 records with different course value, at least one of the record will hit the else clause. Which means that for all 3 of them, at least one 'x' is present, and so the result is 'x' for all 3.

Basically, the first version is: "If the WHEN clause is true for any record, the result is 'o' , otherwise the result is NULL ."

The second version is: "If the WHEN clause is false for any record, the result is 'x' , since that is the highest value, otherwise the result is 'o' ." Or said another way: "If the WHEN clause is true for all records, the result is 'o' , otherwise the result is 'x' ."