COUNT function behavior in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to count all rows in the table named 'employees'.

SQL

SELECT COUNT([1]) FROM employees;

Drag options to blanks, or click blank then click option'

Aname

Bsalary

Cid

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(column_name) when you want to count all rows including NULLs.

Leaving the parentheses empty inside COUNT.

The COUNT(*) counts all rows, including those with NULLs in any column.

2fill in blank

medium

Complete the code to count only the non-NULL values in the 'salary' column from 'employees'.

SQL

SELECT COUNT([1]) FROM employees;

Drag options to blanks, or click blank then click option'

A*

Bsalary

CNULL

DDISTINCT salary

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(*) when only non-NULL values are needed.

Using COUNT(NULL) which always returns 0.

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

3fill in blank

hard

Fix the error in the code to count distinct non-NULL 'department' values in 'employees'.

SQL

SELECT COUNT([1]) FROM employees;

Drag options to blanks, or click blank then click option'

ADISTINCT department

Bdepartment

C*

DALL department

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(department) which counts all non-NULL but not distinct values.

Using COUNT(*) which counts all rows regardless of department.

Use COUNT(DISTINCT column) to count unique non-NULL values.

4fill in blank

hard

Fill both blanks to count how many employees have a non-NULL 'email' and a salary greater than 50000.

SQL

SELECT COUNT([1]) FROM employees WHERE [2] > 50000;

Drag options to blanks, or click blank then click option'

Aemail

Bsalary

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(*) which counts all rows regardless of email NULLs.

Using WHERE email > 50000 which is invalid.

COUNT(email) counts non-NULL emails. The WHERE clause filters salaries greater than 50000.

5fill in blank

hard

Fill all three blanks to count distinct non-NULL 'manager_id' values for employees with 'status' = 'active'.

SQL

SELECT COUNT([1]) FROM employees WHERE [2] = [3];

Drag options to blanks, or click blank then click option'

ADISTINCT manager_id

Bstatus

C'active'

Dmanager_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(manager_id) which counts duplicates.

Using WHERE status = active without quotes causing syntax error.

COUNT(DISTINCT manager_id) counts unique non-NULL managers. The WHERE clause filters active employees.

Practice

1. What does the SQL function COUNT(*) do when used in a query?

easy

A. Counts only rows with NULL values

B. Counts only rows where all columns are NOT NULL

C. Counts all rows in the table, including those with NULL values

D. Counts only distinct values in a column

Solution

1. Step 1: Understand COUNT(*) behavior

   The COUNT(*) function counts every row in the table regardless of NULL values in any column.

2. Step 2: Compare with other COUNT variants

   Unlike COUNT(column), which skips NULLs, COUNT(*) includes all rows.

3. Final Answer:

   Counts all rows in the table, including those with NULL values -> Option C

4. Quick Check:

   COUNT(*) counts all rows [OK]

Hint: COUNT(*) counts every row, NULL or not [OK]

Common Mistakes:

• Thinking COUNT(*) skips NULL rows
• Confusing COUNT(*) with COUNT(column)
• Assuming COUNT(*) counts distinct values

2. Which of the following is the correct syntax to count only non-NULL values in the column age from the table persons?

easy

A. SELECT COUNT(*) FROM persons;

B. SELECT COUNT(age) FROM persons;

C. SELECT COUNT(DISTINCT age) FROM persons;

D. SELECT COUNT(age IS NOT NULL) FROM persons;

Solution

1. Step 1: Identify how to count non-NULL values

   COUNT(column) counts only non-NULL values in that column.

2. Step 2: Check syntax correctness

   SELECT COUNT(age) FROM persons; correctly counts non-NULL age values.

3. Final Answer:

   SELECT COUNT(age) FROM persons; -> Option B

4. Quick Check:

   COUNT(column) counts non-NULL values [OK]

Hint: Use COUNT(column) to count non-NULL values [OK]

Common Mistakes:

• Using COUNT(*) to count non-NULL values
• Using WHERE clause unnecessarily
• Using COUNT with boolean expressions

3. Given the table employees with the column department containing values: ['HR', 'IT', NULL, 'IT', 'HR', 'Finance', NULL], what is the result of the query SELECT COUNT(DISTINCT department) FROM employees;?

medium

A. 3

B. 5

C. 7

D. 4

Solution

1. Step 1: Identify distinct non-NULL values in department

   The distinct non-NULL values are 'HR', 'IT', and 'Finance'. That's 3 unique values.

2. Step 2: Understand COUNT(DISTINCT) behavior

   COUNT(DISTINCT column) counts unique non-NULL values only, so NULLs are excluded.

3. Step 3: Count distinct values

   There are 3 distinct non-NULL values, so the count is 3.

4. Final Answer:

   3 -> Option A

5. Quick Check:

   COUNT(DISTINCT) excludes NULLs [OK]

Hint: COUNT(DISTINCT) counts unique non-NULL values only [OK]

Common Mistakes:

• Including NULL as a distinct value
• Counting total rows instead of distinct
• Confusing COUNT(*) with COUNT(DISTINCT)

4. Consider the query: SELECT COUNT(employee_id) FROM staff; but it returns 0 even though the table has rows. What is the most likely reason?

medium

A. The column employee_id contains only NULL values

B. The table staff is empty

C. COUNT(employee_id) counts all rows including NULLs

D. Syntax error in the query

Solution

1. Step 1: Understand COUNT(column) behavior

   COUNT(column) counts only non-NULL values in that column.

2. Step 2: Analyze why count is zero

   If all employee_id values are NULL, COUNT returns 0 even if rows exist.

3. Final Answer:

   The column employee_id contains only NULL values -> Option A

4. Quick Check:

   COUNT(column) excludes NULLs [OK]

Hint: COUNT(column) returns 0 if all values are NULL [OK]

Common Mistakes:

• Assuming COUNT(column) counts all rows
• Thinking table is empty without checking data
• Assuming syntax error without checking query

5. You want to find how many unique customers placed orders, but some orders have NULL customer IDs. Which query correctly counts unique customers excluding NULLs?

hard

A. SELECT COUNT(customer_id) FROM orders;

B. SELECT COUNT(*) FROM orders WHERE customer_id IS NOT NULL;

C. SELECT COUNT(DISTINCT *) FROM orders;

D. SELECT COUNT(DISTINCT customer_id) FROM orders;

Solution

1. Step 1: Understand requirement for unique customers excluding NULLs

   We need to count distinct customer IDs ignoring NULLs.

2. Step 2: Analyze options

   SELECT COUNT(DISTINCT customer_id) FROM orders; uses COUNT(DISTINCT customer_id), which counts unique non-NULL values correctly.

3. Step 3: Eliminate incorrect options

   SELECT COUNT(customer_id) FROM orders; counts all non-NULL customer IDs including duplicates. SELECT COUNT(*) FROM orders WHERE customer_id IS NOT NULL; counts rows with non-NULL customer IDs but not distinct. SELECT COUNT(DISTINCT *) FROM orders; is invalid syntax.

4. Final Answer:

   SELECT COUNT(DISTINCT customer_id) FROM orders; -> Option D

5. Quick Check:

   COUNT(DISTINCT column) counts unique non-NULL values [OK]

Hint: Use COUNT(DISTINCT column) for unique non-NULL counts [OK]

Common Mistakes:

• Using COUNT(column) to count unique values
• Using COUNT(*) with WHERE instead of DISTINCT
• Using invalid syntax like COUNT(DISTINCT *)