Bird
Raised Fist0
SQLquery~3 mins

COUNT(*) vs COUNT(column) difference in SQL - When to Use Which

Choose your learning style10 modes available
LearnWhyDeepVisualTryChallengeProjectRecallTime

Start learning this pattern below

Jump into concepts and practice - no test required

or
Recommended
Test this pattern10 questions across easy, medium, and hard to know if this pattern is strong
The Big Idea

Discover how a tiny difference in counting can change your data insights completely!

The Scenario

Imagine you have a big list of customer orders in a spreadsheet. You want to know how many orders there are in total and how many orders actually have a delivery date filled in. Doing this by hand means scanning each row and counting carefully.

The Problem

Counting manually is slow and easy to mess up, especially if some orders don't have delivery dates. You might count empty cells by mistake or miss some rows. It's hard to keep track and get accurate numbers quickly.

The Solution

Using SQL's COUNT(*) and COUNT(column) lets the database do the counting perfectly and fast. COUNT(*) counts all rows, while COUNT(column) counts only rows where that column has a non-NULL value. This removes guesswork and speeds up your work.

Before vs After
✗ Before
count = 0
for row in orders:
  if row.delivery_date is not None:
    count += 1
✓ After
SELECT COUNT(*) FROM orders;
SELECT COUNT(delivery_date) FROM orders;
What It Enables

You can quickly get total counts and counts of specific data presence, helping you understand your data better and make decisions faster.

Real Life Example

A store manager wants to know how many orders were placed (total rows) and how many have been shipped (delivery date filled). Using COUNT(*) and COUNT(delivery_date) gives these numbers instantly.

Key Takeaways

COUNT(*) counts all rows, including those with empty columns.

COUNT(column) counts only rows where that column is not NULL.

Using these functions saves time and avoids manual counting errors.

Practice

(1/5)
1. What is the main difference between COUNT(*) and COUNT(column_name) in SQL?
easy
A. COUNT(*) counts only rows where all columns are NOT NULL, COUNT(column_name) counts all rows.
B. COUNT(*) and COUNT(column_name) always return the same result.
C. COUNT(*) counts only NULL values, COUNT(column_name) counts non-NULL values.
D. COUNT(*) counts all rows, while COUNT(column_name) counts only rows where the column is NOT NULL.

Solution

  1. Step 1: Understand COUNT(*)

    COUNT(*) counts every row in the table, including those with NULL values in any column.

  2. Step 2: Understand COUNT(column_name)

    COUNT(column_name) counts only rows where the specified column is NOT NULL, ignoring rows where that column is NULL.

  3. Final Answer:

    COUNT(*) counts all rows; COUNT(column_name) counts only non-NULL values in that column. -> Option D

  4. Quick Check:

    COUNT(*) counts all rows, COUNT(column) skips NULLs [OK]
Hint: Remember: * counts all rows, column counts non-NULL only [OK]
Common Mistakes:
  • Thinking COUNT(column) counts NULL values
  • Assuming COUNT(*) ignores NULLs
  • Believing both always return same count
2. Which of the following SQL queries correctly counts the number of rows where the column email is NOT NULL?
easy
A. SELECT COUNT(email) FROM users;
B. SELECT COUNT(*) FROM users WHERE email IS NULL;
C. SELECT COUNT(email) FROM users WHERE email IS NULL;
D. SELECT COUNT(*) FROM users;

Solution

  1. Step 1: Analyze COUNT(email)

    COUNT(email) counts only rows where email is NOT NULL, so it already filters NULLs.

  2. Step 2: Check the WHERE clause necessity

    Adding WHERE email IS NOT NULL is redundant with COUNT(email), so SELECT COUNT(email) FROM users; is correct and simpler.

  3. Final Answer:

    SELECT COUNT(email) FROM users; -> Option A

  4. Quick Check:

    COUNT(column) counts non-NULL rows without WHERE [OK]
Hint: COUNT(column) counts non-NULL rows without extra WHERE [OK]
Common Mistakes:
  • Adding unnecessary WHERE clause with COUNT(column)
  • Using COUNT(*) with wrong WHERE condition
  • Confusing NULL and NOT NULL filters
3. Given the table orders with 5 rows where the discount column has values: 10, NULL, 5, NULL, 0, what will be the result of SELECT COUNT(*) AS total, COUNT(discount) AS discount_count FROM orders;?
medium
A. total = 5, discount_count = 3
B. total = 5, discount_count = 5
C. total = 3, discount_count = 3
D. total = 3, discount_count = 5

Solution

  1. Step 1: Count total rows with COUNT(*)

    COUNT(*) counts all 5 rows regardless of NULLs.

  2. Step 2: Count non-NULL discount values with COUNT(discount)

    Only 3 rows have non-NULL discount values (10, 5, 0), so discount_count is 3.

  3. Final Answer:

    total = 5, discount_count = 3 -> Option A

  4. Quick Check:

    COUNT(*) = all rows, COUNT(column) = non-NULL rows [OK]
Hint: COUNT(*) counts all rows; COUNT(column) skips NULLs [OK]
Common Mistakes:
  • Counting NULLs in COUNT(column)
  • Confusing total rows with non-NULL counts
  • Assuming 0 is NULL
4. You wrote this query: SELECT COUNT(column_name) FROM table_name; but it returns 0. The column has some NULL and some non-NULL values. What is the most likely problem?
medium
A. COUNT(column_name) counts NULL values, so it should not return 0.
B. The column_name is misspelled or does not exist in the table.
C. COUNT(*) should be used instead of COUNT(column_name) to count non-NULL values.
D. The table is empty, so COUNT(column_name) returns 0.

Solution

  1. Step 1: Check column existence

    If COUNT(column_name) returns 0 but column has non-NULL values, likely the column name is wrong or missing.

  2. Step 2: Understand COUNT behavior

    COUNT(column_name) counts non-NULL values; if column exists and has non-NULLs, result won't be zero.

  3. Final Answer:

    Column name is misspelled or does not exist. -> Option B

  4. Quick Check:

    Wrong column name causes zero count [OK]
Hint: Check column spelling if COUNT(column) returns zero unexpectedly [OK]
Common Mistakes:
  • Assuming COUNT(column) counts NULLs
  • Using COUNT(*) when column is misspelled
  • Ignoring empty table possibility without checking
5. You have a table employees with 100 rows. The phone_number column has 80 non-NULL values and 20 NULLs. You want to find how many employees have a phone number and how many total employees there are. Which query gives both counts correctly?
hard
A. SELECT COUNT(phone_number) + COUNT(*) AS total FROM employees;
B. SELECT COUNT(*) AS with_phone, COUNT(phone_number) AS total_employees FROM employees;
C. SELECT COUNT(phone_number) AS with_phone, COUNT(*) AS total_employees FROM employees;
D. SELECT COUNT(phone_number) AS total_employees, COUNT(*) AS with_phone FROM employees;

Solution

  1. Step 1: Count employees with phone numbers

    COUNT(phone_number) counts only non-NULL phone numbers, so it returns 80.

  2. Step 2: Count total employees

    COUNT(*) counts all rows, so it returns 100.

  3. Final Answer:

    SELECT COUNT(phone_number) AS with_phone, COUNT(*) AS total_employees FROM employees; -> Option C

  4. Quick Check:

    COUNT(phone_number) = 80, COUNT(*) = 100 [OK]
Hint: Use COUNT(column) for non-NULL, COUNT(*) for total rows [OK]
Common Mistakes:
  • Swapping counts in SELECT clause
  • Adding counts instead of separate columns
  • Confusing which count counts NULLs