Consider a table orders with columns order_id (unique), customer_id, and coupon_code (nullable). Which query correctly counts the number of orders that used a coupon code?

hard📝 Application Q9 of 15

SQL - Aggregate Functions

Consider a table orders with columns order_id (unique), customer_id, and coupon_code (nullable). Which query correctly counts the number of orders that used a coupon code?

ASELECT COUNT(*) FROM orders WHERE coupon_code IS NOT NULL;

BBoth B and D

CSELECT COUNT(*) FROM orders WHERE coupon_code = NULL;

DSELECT COUNT(coupon_code) FROM orders;

Step-by-Step Solution

Solution:

Step 1: Analyze COUNT(coupon_code)
Counts rows where coupon_code is NOT NULL.
Step 2: Analyze COUNT(*) with WHERE coupon_code IS NOT NULL
Also counts rows where coupon_code is NOT NULL.
Step 3: Check invalid option
coupon_code = NULL is invalid; must use IS NULL.
Final Answer:
Both B and D -> Option B
Quick Check:
COUNT(column) and COUNT(*) with IS NOT NULL match [OK]

Quick Trick: COUNT(column) = COUNT(*) with WHERE column IS NOT NULL [OK]

Common Mistakes:

MISTAKES

Using = NULL instead of IS NULL
Thinking COUNT(column) and COUNT(*) differ here
Ignoring NULL filtering in WHERE

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More SQL Quizzes

Consider a table orders with columns order_id (unique), customer_id, and coupon_code (nullable). Which query correctly counts the number of orders that used a coupon code?

Step 1: Analyze COUNT(coupon_code)

Step 2: Analyze COUNT(*) with WHERE coupon_code IS NOT NULL

Step 3: Check invalid option

Final Answer:

Quick Check:

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