SQL - Aggregate Functions
Which of the following is the correct syntax to count distinct non-NULL values in the column
customer_id from the table sales?Which of the following is the correct syntax to count distinct non-NULL values in the column customer_id from the table sales?
easy📝 Syntax Q3 of 15
Step-by-Step Solution
Solution:
Step 1: Recall syntax for counting distinct values
The correct syntax is COUNT(DISTINCT column_name) without extra parentheses.Step 2: Check each option
SELECT COUNT(DISTINCT customer_id) FROM sales; uses correct syntax; A and C are invalid; D has extra parentheses which is invalid in standard SQL.Final Answer:
SELECT COUNT(DISTINCT customer_id) FROM sales; -> Option DQuick Check:
COUNT(DISTINCT column) syntax is correct [OK]
Quick Trick: Use COUNT(DISTINCT column) to count unique non-NULL values [OK]
Common Mistakes:
MISTAKES- Placing DISTINCT after column name
- Using COUNT(DISTINCT) without column
- Adding extra parentheses around column
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