[Solved] Consider a UNIQUE index on columns first_name and last_name in table people.... Which insert will violate the UNIQUE constraint?INSERT INTO people (first_name, last_name) VALUES ('John', 'Doe'); | SQL

SQL - Indexes and Query Performance

Consider a UNIQUE index on columns first_name and last_name in table people. Which insert will violate the UNIQUE constraint?

INSERT INTO people (first_name, last_name) VALUES ('John', 'Doe');
INSERT INTO people (first_name, last_name) VALUES ('John', 'Smith');
INSERT INTO people (first_name, last_name) VALUES ('John', 'Doe');

AOnly the second insert violates the UNIQUE constraint

BOnly the third insert violates the UNIQUE constraint

CBoth second and third inserts violate the UNIQUE constraint

DNone of the inserts violate the UNIQUE constraint

Step-by-Step Solution

Solution:

Step 1: Understand UNIQUE index on multiple columns
The UNIQUE index applies to the combination of first_name and last_name together, not individually.
Step 2: Check each insert's combination
The first insert adds ('John', 'Doe'). The second inserts ('John', 'Smith') which is unique. The third repeats ('John', 'Doe'), violating the UNIQUE constraint.
Final Answer:
Only the third insert violates the UNIQUE constraint -> Option B
Quick Check:
UNIQUE on multiple columns = unique combinations [OK]

Quick Trick: UNIQUE on multiple columns means unique pairs [OK]

Common Mistakes:

Thinking each column is unique separately
Assuming second insert duplicates first
Ignoring combined uniqueness rule

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Consider a UNIQUE index on columns first_name and last_name in table people. Which insert will violate the UNIQUE constraint?

Step 1: Understand UNIQUE index on multiple columns

Step 2: Check each insert's combination

Final Answer:

Quick Check:

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