SQL - CASE Expressions
How do you correctly write a query to count the number of rows where
age is less than 30 using CASE inside COUNT?How do you correctly write a query to count the number of rows where age is less than 30 using CASE inside COUNT?
easy📝 Syntax Q3 of 15
Step-by-Step Solution
Solution:
Step 1: Use CASE with WHEN condition
The CASE statement should specify WHEN age < 30 THEN 1 ELSE NULL to count only matching rows.Step 2: Use COUNT to count non-null values
COUNT counts non-null values, so only rows where age < 30 contribute to the count.Final Answer:
SELECT COUNT(CASE WHEN age < 30 THEN 1 ELSE NULL END) FROM people; -> Option DQuick Check:
COUNT ignores NULLs, so only age < 30 rows counted [OK]
Quick Trick: COUNT counts non-null values only [OK]
Common Mistakes:
- Using ELSE 0 causes all rows to be counted
- Omitting WHEN keyword in CASE
- Returning strings instead of numbers in CASE
Master "CASE Expressions" in SQL
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