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SQLquery~5 mins

COUNT(*) vs COUNT(column) difference in SQL

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Introduction
COUNT(*) counts all rows, while COUNT(column) counts only rows where the column has a value. This helps you understand data completeness.
When you want to know how many rows are in a table regardless of missing data.
When you want to count only rows where a specific column has a value (not NULL).
When checking how many entries have data in a particular column.
When comparing total rows versus rows with valid data in a column.
When summarizing data completeness in reports.
Syntax
SQL
SELECT COUNT(*) FROM table_name;
SELECT COUNT(column_name) FROM table_name;
COUNT(*) counts every row, including those with NULLs in any column.
COUNT(column_name) counts only rows where column_name is NOT NULL.
Examples
Counts all rows in the employees table.
SQL
SELECT COUNT(*) FROM employees;
Counts only rows where the email column is not NULL.
SQL
SELECT COUNT(email) FROM employees;
Shows total rows and how many have a phone number.
SQL
SELECT COUNT(*) AS total_rows, COUNT(phone) AS phone_count FROM contacts;
Sample Program
This creates a table with some NULL emails and counts total rows and rows with emails.
SQL
CREATE TABLE people (
  id INT,
  name VARCHAR(50),
  email VARCHAR(50)
);

INSERT INTO people (id, name, email) VALUES
(1, 'Alice', 'alice@example.com'),
(2, 'Bob', NULL),
(3, 'Charlie', 'charlie@example.com'),
(4, 'Diana', NULL);

SELECT COUNT(*) AS total_rows, COUNT(email) AS email_count FROM people;
OutputSuccess
Important Notes
COUNT(*) is faster when you want total rows because it doesn't check column values.
COUNT(column) ignores NULLs, so it helps find how many rows have actual data in that column.
NULL means no value, so COUNT(column) skips those rows.
Summary
COUNT(*) counts all rows, including those with NULLs.
COUNT(column) counts only rows where the column is NOT NULL.
Use COUNT(column) to find how many rows have data in that column.

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