[Solved] You want to ensure that the combination of first_name and last_name in the employees table is unique, but duplicates are allowed in each column... Which SQL statement correctly enforces this? | SQL

SQL - Table Constraints

You want to ensure that the combination of first_name and last_name in the employees table is unique, but duplicates are allowed in each column individually. Which SQL statement correctly enforces this?

AALTER TABLE employees ADD UNIQUE (first_name, last_name);

BALTER TABLE employees ADD UNIQUE (first_name); ALTER TABLE employees ADD UNIQUE (last_name);

CALTER TABLE employees ADD UNIQUE first_name, last_name;

DALTER TABLE employees ADD UNIQUE CONSTRAINT employees_unique (first_name);

Step-by-Step Solution

Solution:

Step 1: Understand multi-column UNIQUE and analyze options
A UNIQUE constraint on multiple columns ensures the combination is unique (individual duplicates allowed). "ALTER TABLE employees ADD UNIQUE (first_name, last_name);" correctly adds UNIQUE on the pair. Adding UNIQUE separately disallows individual duplicates; other syntax is wrong or ignores one column.
Final Answer:
ALTER TABLE employees ADD UNIQUE (first_name, last_name); -> Option A
Quick Check:
Multi-column UNIQUE = unique pairs [OK]

Quick Trick: Use UNIQUE on multiple columns for combined uniqueness [OK]

Common Mistakes:

MISTAKES

Adding UNIQUE on each column separately
Using incorrect syntax without parentheses
Ignoring the need for combined uniqueness

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Which SQL statement correctly enforces this?

Step 1: Understand multi-column UNIQUE and analyze options

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