# Domain Relational Calculus in DBMS

Domain Relational Calculus is a non-procedural query language equivalent in power to Tuple Relational Calculus. Domain Relational Calculus provides only the description of the query but it does not provide the methods to solve it. In Domain Relational Calculus, a query is expressed as,

`{ < x1, x2, x3, ..., xn > | P (x1, x2, x3, ..., xn ) } `

where, < x1, x2, x3, …, xn > represents resulting domains variables and P (x1, x2, x3, …, xn ) represents the condition or formula equivalent to the Predicate calculus.

Predicate Calculus Formula:

1. Set of all comparison operators
2. Set of connectives like and, or, not
3. Set of quantifiers

Example:

Table-1: Customer

Customer name Street City
Sayantan Udaypur Balurghat
Soumya Nutanchati Bankura
Ritu Juhu Mumbai

Table-2: Loan

Loan number Branch name Amount
L01 Main 200
L03 Main 150
L10 Sub 90
L08 Main 60

Table-3: Borrower

Customer name Loan number
Ritu L01
Debomit L08
Soumya L03

Query-1: Find the loan number, branch, amount of loans of greater than or equal to 100 amount.

`{≺l, b, a≻ | ≺l, b, a≻ ∈ loan ∧ (a ≥ 100)}`

Resulting relation:

Loan number Branch name Amount
L01 Main 200
L03 Main 150

Query-2: Find the loan number for each loan of an amount greater or equal to 150.

`{≺l≻ | ∃ b, a (≺l, b, a≻ ∈ loan ∧ (a ≥ 150)}`

Resulting relation:

Loan number
L01
L03

Query-3: Find the names of all customers having a loan at the “Main” branch and find the loan amount .

`{≺c, a≻ | ∃ l (≺c, l≻ ∈ borrower ∧ ∃ b (≺l, b, a≻ ∈ loan ∧ (b = “Main”)))}`

Resulting relation:

Customer Name Amount
Ritu 200
Debomit 60
Soumya 150

Note:
The domain variables those will be in resulting relation must appear before | within ≺ and ≻ and all the domain variables must appear in which order they are in original relation or table.