GATE CSE 2015 | Set 1 | Question: 7

Question

SELECT operation in SQL is equivalent to

• A. The selection operation in relational algebra
• B. The selection operation in relational algebra, except that SELECT in SQL retains duplicates
• C. The projection operation in relational algebra
• D. The projection operation in relational algebra, except that SELECT in SQL retains duplicates

Comments

Detailed Video Solution: https://youtu.be/h3pJZbed9M8?t=5519&feature=shared

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select in sql retains duplicates

select in ra removes duplicates

select in sql = project in ra except sql retains duplicates

 

Answer 1

Option D is correct because SELECT operation in SQL is equivalent to The projection operation in relational algebra, except that SELECT in SQL retains duplicates but projection gives only distinct. 

Comments

The below link has a ppt to explain this answer

http://courses.cs.vt.edu/~cs4604/Fall09/lectures/lecture-03-intro-sql-relational-algebra.pdf

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Selection in relational algebra select rows and projection in RA select columns. Select operation in SQL selects columns but duplicates values are retained in SQL.

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Option D is the the correct answer

Answer 2

Relational algebra eliminate duplicacy    but SQL retain duplicates 

 

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Answer 3

Ans D

In relational algebra, the projection operation (π) selects specific columns and removes duplicates. In SQL, the SELECT statement also selects columns but retains duplicates unless you use DISTINCT. Therefore, the SQL SELECT operation is similar to the projection operation in relational algebra, except that it does not automatically remove duplicates.

In relational algebra, the selection operation (σ) filters rows based on a specified condition, similar to the WHERE clause in SQL. It does not affect columns or remove duplicates. For example, σ(Major='Math')(Students) would select all rows from the Students table where the Major is 'Math'.

Answer 4

The SELECT operation in SQL is equivalent to the selection operation in relational algebra.

The SELECT operation in SQL is used to retrieve specific columns and rows from a table in a relational database. This operation is similar to the selection operation in relational algebra, which is used to select a subset of rows from a relation based on a specified condition.

Both SELECT in SQL and selection in relational algebra retain duplicate rows when they are selected, and the SELECT operation in SQL does not eliminate duplicates.

The projection operation in relational algebra is used to select a subset of columns from a relation, which is not the same as the SELECT operation in SQL.