Consider the table orders with columns order_id (PRIMARY KEY) and tracking_number with a UNIQUE constraint. What will this query return?

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SQL - Table Constraints

Consider the table orders with columns order_id (PRIMARY KEY) and tracking_number with a UNIQUE constraint. What will this query return?

SELECT tracking_number FROM orders WHERE tracking_number = 'ABC123';

Assuming there is exactly one row with tracking_number = 'ABC123'.

AMultiple rows with tracking_number 'ABC123'

BOne row with tracking_number 'ABC123'

CNo rows, because UNIQUE disallows selection

DError due to UNIQUE constraint

Step-by-Step Solution

Solution:

Step 1: Understand UNIQUE constraint effect on data
UNIQUE ensures no duplicates, so only one row can have 'ABC123'.
Step 2: Analyze SELECT query result
The query returns the single row matching the tracking number.
Final Answer:
One row with tracking_number 'ABC123' -> Option B
Quick Check:
UNIQUE allows single matching row in SELECT [OK]

Quick Trick: UNIQUE means SELECT returns at most one matching row [OK]

Common Mistakes:

MISTAKES

Thinking UNIQUE prevents selection
Expecting multiple rows with same value
Confusing UNIQUE with NOT NULL

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More SQL Quizzes

Consider the table orders with columns order_id (PRIMARY KEY) and tracking_number with a UNIQUE constraint. What will this query return?

Step 1: Understand UNIQUE constraint effect on data

Step 2: Analyze SELECT query result

Final Answer:

Quick Check:

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