SQL Builder Error – Incompatible Types

Hey techyv experts,
I am new to SQL and trying to learn everything about it. When I was trying to generate SQL Statements using the SQL Builder and selected two join columns, I encountered this error message:
Incompatible Types
IWAQ0004W Join types not compatible
The type of column DEPTNO is CHAR
The type of column SALARY is DECIMAL
When I tried to use the drag-and-drop method to create a join directly from the Table window by hovering over the desired join column on one table and releasing the mouse button on the join column on the target table, this error message still appeared.
What could be the cause of this and how can I avoid this kind of error? I hope someone can help me. Any help would be appreciated.
Thanks.
