hi, i have this tables

Table A
{
INTEGER ID; //PK
INTEGER TARGET; //NULLABLE FK TO TABLE B
}

Table B
{
INTEGER ID; //PK
INTEGER VALUE;
}

i need to implement a store procedure with the integer THRESHOLD as parameter, in this procedure i need to select all the record of table A wich have Target NULL or where the joined table B record has VALUE < THRESHOLD.

i'm not very good in sql and i canot find a solution, can samebody help me?
thanks