SQL - Table Constraints
You want to ensure a
username column is unique and never empty. Which combination of constraints should you apply?You want to ensure a username column is unique and never empty. Which combination of constraints should you apply?
hard📝 Application Q8 of 15
Step-by-Step Solution
Solution:
Step 1: Identify constraints for uniqueness and non-empty
UNIQUE ensures no duplicates; NOT NULL prevents empty (NULL) values.Step 2: Evaluate other options
PRIMARY KEY could work but may not be suitable if username is not the key; CHECK alone doesn't prevent NULL; DEFAULT '' allows empty string but not NULL.Final Answer:
UNIQUE and NOT NULL -> Option DQuick Check:
Unique + Not Null = Unique, non-empty usernames [OK]
Quick Trick: Combine UNIQUE and NOT NULL for unique, filled columns [OK]
Common Mistakes:
MISTAKES- Using only CHECK without NOT NULL
- Assuming DEFAULT prevents NULL
- Confusing PRIMARY KEY with uniqueness requirement
Master "Table Constraints" in SQL
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