[MySQL] NULL -> 0 변경. COALESCE, IFNULL

# COALESCE() 
: 주어진 인수에서 첫 번째가 NULL 이 아닌 값을 반환.
> SELECT COALESCE(value1);   
> SELECT COALESCE(value1, value2);
> SELECT COALESCE(value1, value2, value3);

# IFNULL() 
: expr1 값이 NULL이 아니면 expr1 의 값을, NULL 이면 expr2 의 값을 반환.
> SELECT NULLIF(expr1, expr2);
> SELECT NULLIF(max(expr1), expr2);


# NULLIF() 
: IFNULL의 반대. NULL 이면 NULL 그대로 반환한다. 
  expr1 과 expr2 이 같거나 참이면 NULL(또는 1)를 반환하고, 아니면 expr1 를 반환.
> SELECT IFNULL(expr1, expr2);




참고 : 
https://kingscode.tistory.com/340
https://www.habonyphp.com/2019/02/coalesce.html
https://www.habonyphp.com/2019/02/nullif.html