The select operation:

Question

BlackTom AI · Accepted Answer

The select operation retrieves rows (tuples) from a relation that satisfy a specified predicate. In relational algebra this is written σ_condition(R) and is defined as σ_condition(R) = { t ∈ R | condition(t) } — i.e., every tuple t in relation R for which the condition is true is included in the result.

Relevant knowledge points:
- Relational algebra distinguishes selection (σ) and projection (π). Selection chooses rows; projection chooses columns.
- SQL’s WHERE clause corresponds to relational selection; SQL’s SELECT clause corresponds to projection (columns).
- Other basic operations include join (combining tables), and DDL/DML statements that define or modify database content/structure.

Step-by-step reasoning with a concrete example:
1. Suppose a table Employees has tuples:
   E1 = (Alice, Sales, salary=60000)
   E2 = (Bob, HR, salary=45000)
   E3 = (Carol, Sales, salary=70000)
2. Apply the selection σ_salary>50000(Employees).
3. Evaluate condition for each tuple:
   - E1: 60000 > 50000 → true → include E1
   - E2: 45000 > 50000 → false → exclude E2
   - E3: 70000 > 50000 → true → include E3
4. Result = {E1, E3} — a subset of all records meeting the stated criteria.

Why the correct answer is correct:
- The provided correct option explicitly describes selecting records that meet criteria (rows), exactly matching the formal definition of selection.

Why the other choices are incorrect:
- "creates a subset consisting of columns in a table." describes projection, not selection.
- "identifies the table from which the columns will be selected." describes the FROM clause or relation reference, not selection.
- "combines relational tables..." describes join operations.
- "specifies the content of the database." is vague and more related to data definition or population, not the select operation.

Therefore, the correct answer is: creates a subset consisting of all records in the file that meet stated criteria. This matches the provided answer.

The select operation: 单项选择题

类似问题

Consider the following statements, select your best answer.

The project operation:

Which of the following is TRUE?

RA For each of the relational algebra below, state whether they are correct or not for the following statement: List the topics of forums where the general user with user id “1” has posted at least once before the date “01-01-2022”. 1: A 2: B 3: C 4: D

Match the following terms and definitions that are used in relational algebra: 1: SELECT 2: PROJECT 3: Union-Compatible 4: UNION 5: JOIN

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