COUNT(*) vs COUNT(column) difference in SQL - Quick Revision & Key Differences

Recall & Review

beginner

What does COUNT(*) do in SQL?

COUNT(*) counts all rows in a table, including rows with NULL values in any column.

beginner

What does COUNT(column) do in SQL?

COUNT(column) counts only the rows where the specified column is NOT NULL.

intermediate

How does COUNT(*) differ from COUNT(column) when the column contains NULL values?

COUNT(*) counts every row regardless of NULL values, but COUNT(column) skips rows where the column is NULL.

beginner

If a table has 10 rows and 3 rows have NULL in column age, what will COUNT(*) and COUNT(age) return?

COUNT(*) returns 10 because it counts all rows.
COUNT(age) returns 7 because it counts only rows where age is NOT NULL.

beginner

Why might you choose COUNT(column) over COUNT(*)?

Use COUNT(column) when you want to count only rows with meaningful (non-NULL) data in that column.

What does COUNT(*) count in a SQL table?

A Only rows with NULL values

B Only rows where a specific column is NOT NULL

C All rows including those with NULL values

D Only distinct rows

COUNT(*) counts every row in the table, regardless of NULL values.

What will COUNT(column) ignore when counting?

A All rows

B Rows where the column is NULL

C Rows where the column is NOT NULL

D Only duplicate rows

COUNT(column) counts only rows where the column is NOT NULL, so it ignores NULLs.

If a column has no NULL values, how do COUNT(*) and COUNT(column) compare?

A They return zero

B <code>COUNT(*)</code> returns more

C <code>COUNT(column)</code> returns more

D They return the same count

When there are no NULLs, both counts return the total number of rows.

Which COUNT function should you use to count all rows regardless of NULLs?

A COUNT(*)

B COUNT(column)

C COUNT(DISTINCT column)

D COUNT(NULL)

COUNT(*) counts all rows including those with NULLs.

What does COUNT(column) return if all values in the column are NULL?

A 0

B Number of rows

C NULL

D Error

COUNT(column) returns 0 if all values are NULL because it counts only non-NULL values.

Explain the difference between COUNT(*) and COUNT(column) in SQL.

Think about how NULL values affect counting.

When would you use COUNT(column) instead of COUNT(*)?

Consider what NULL means in your data.

Practice

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