MADE EASY TEST SERIES

Question

1. Unique not null is equivalent to primary key.
2. Relational Algebra and SQL has same expressive power.

Which of the above statements are False?

Comments

1. TRUE, if the given set is minimal then the necessary conditions for Primary is that it should be unique and not NULL
2. FALSE, as aggregate functions of SQL cannot be expressed in RA
Let me know if I am wrong

Answer 1

1. TRUE, ​​​​​​Primary constraints are Not NULL and it should be unique key
2. FALSE, SQL has more power than Relational Algebra because basic relational algebra query cannot perform mathematical aggregate functions

References; 

GROUP BY, According to the book Fundamentals of Database Systems (Elmasri, Navathe 2011 6th ed):

Another type of request that cannot be expressed in the basic relational algebra is to specify mathematical aggregate functions on collections of values from the database.
...
We can define an AGGREGATE FUNCTION operation, using the symbol ℑ (pronounced script F)7, to specify these types of requests as follows:

<grouping attributes> ℑ <function list> (R)

where <grouping attributes> is a list of attributes of the relation specified in R, and <function list> is a list of (<function> <attribute>) pairs. In each such pair, <function> is one of the allowed functions—such as SUM, AVERAGE, MAXIMUM, MINIMUM,COUNT—and <attribute> is an attribute of the relation specified by R. The resulting relation has the grouping attributes plus one attribute for each element in the function list.

In addition to it,

• Relational algebra has same power as safe TRC and safe DRC
• And TRC has more power than Relational algebra because of unsafe queries.

Comments

But in solution it is given that data dictionary follows different structure for primary key and unique,not null. So I am confused.

---

There can be multiple unique+Not null but only one primary key, although your answer is technically correct but the question is ambiguous.