Insert null in INTEGER COLUMN in mysql

In my mysql database we have few nullable integer and date columns.

MYSQL Join 2 Columns to one column with different values

I have a problem while trying left join one column from one table, because i need 2 columns consult the name by the id

unable to understand mysqli methods and properties, some uses “( )” an some dont. why?

i am using mysqli oop with php and seen multiple methods inside mysqli, some of them have () and some dont have (). why so?

What am I missing here when creating this Foreign Key?

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