Question

project assignment is n to n relationship where n can be zero.

pid is primary key for the project table

eid is primary key for the employee table

a project may not be assigned to any employee.similary an employee may not have any project in his hand.

how to write this query? eid should take value as null or the value from the emp table.
pid should either take null or the value from the table project.

IS IT CORRECT.

CREATE TABLE Proj_Assign
(
eid VARCHAR(25),
pid VARCHAR(25),
PRIMARY KEY(eid,pid),
FOREIGN KEY eid REFERENCES employee(eid),
FOREIGN KEY pid REFERENCES project(pid)
);

Answer 1

Give this way:

FOREIGN KEY (t_eid) REFERENCES parent(eid)
    ON DELETE CASCADE

Instead of using same eid twice. It is explained in the documentation.

Also, the original parent table should be there before you create this table and you need to manually insert the queries this way:

INSERT INTO `Proj_Assign` (`eid`, `pid`) VALUES (1, 1);

Whatever query you have written is right as per documentation.