Which level of locking provides the highest degree of concurrency in a relational database?

Let R₁ (A, B, C) and R₂ (D, E) be two relation schema, where the primary keys are shown underlined, and let C be a foreign key in R₁ referring to R₂. Suppose there is no violation of the above referential integrity constraint in the corresponding relation instances r₁ and r₂. Which one of the following relational algebra expressions would necessarily produce an empty relation?

Consider two tables in a relational database with columns and rows as follows:

 Table: Student

Roll_no	Name	Dept_id
1	ABC	1
2	DEF	1
3	GHI	2
4	JKL	3

 Table: Department

Dept_id	Dept_name
1	A
2	B
3	C

Roll_no is the primary key of the Student table, Dept_id is the primary key of the Department table and Studetn.Dept_id is a foreign key from
Department. Dept_id.

What will happen if we try to execute the following two SQL statements?

(i) update Student set Dept_id = Null where Roll_no =1
(ii) update Department set Dept_id = Null where Dept_id =1

A relational database contains two table student and department in which student table has columns roll_no, name and dept_id and department table has columns dept_id and detp_name. the following insert statements were executed successfully to populate the empty tables:

 Insert into department values (1, ‘Mathematics’)
 Insert into department values (2, ‘Physics’)
 Insert into student values (1, ‘Navin’,1)
 Insert into student values (2, ‘Mukesh’,2)
 Insert into student values (3, ‘Gita’,1)

How many rows and columns will be retrieved by the following SQL statement?

Select * from student, department;