Which of the following relational algebraic expression is equivalent to the SQL query SELECT X FROM B WHERE Y?

Correct option is D

The SQL query is:
SELECT X
FROM B
WHERE Y;
We need to translate this SQL query into relational algebra.
Step 1: Understand the WHERE clause
The WHERE Y condition filters the rows of relation B based on condition Y.
In relational algebra, selection is represented by: $$\sigma$$​​
Therefore, $$\sigma_Y(B)$$​ means select those tuples from B that satisfy condition Y.
Step 2: Understand the SELECT X clause
The SQL statement:
SELECT X
means that after filtering, we want only attribute X.
In relational algebra, projection is represented by: π
Therefore, $$\pi_X(\sigma_Y(B))$$​ means:
1. First select the tuples satisfying Y.
2. Then project attribute X.
Thus:
​$$\boxed{\pi_X(\sigma_Y(B))}$$​​
Information Booster
1. Selection $$(\sigma)$$​ → selects rows/tuples satisfying a condition.
2. Projection $$(\pi)$$​ → selects columns/attributes.
3. SQL WHERE corresponds primarily to selection.
4. SQL SELECT of specific attributes corresponds to projection.
5. The usual order for this query is:
​$$\boxed{\text{Selection first, then Projection}}$$​​
Additional Knowledge
The options can be eliminated as follows:
· (a) $$\sigma_Y(\pi_X(B))$$​ — projection is performed before applying the condition; generally not equivalent.
· (b) $$\pi_Y(\sigma_X(B))$$​ — selection and projection attributes are reversed.
· (c) $$\sigma_X(\pi_X(B))$$​ — uses X as the selection condition rather than Y.