NULL in DISTINCT, GROUP BY, and ORDER BY in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does NULL represent in SQL?

NULL means a missing or unknown value. It is not the same as zero or an empty string.

beginner

How does DISTINCT treat NULL values in SQL?

DISTINCT treats all NULLs as the same value. So, multiple NULLs count as one distinct value.

beginner

How does GROUP BY handle NULL values?

GROUP BY groups all rows with NULL in the same column together as one group.

intermediate

Where do NULL values appear when using ORDER BY?

In ORDER BY, NULLs usually appear last when sorting ascending, and first when sorting descending, but this can vary by database.

intermediate

Can NULL values affect the result count in DISTINCT or GROUP BY?

Yes, since all NULLs count as one value, they reduce the number of distinct groups or rows compared to counting each NULL separately.

In SQL, how does DISTINCT treat multiple NULL values in a column?

A Counts all NULLs as one distinct value

B Ignores NULL values completely

C Returns an error if NULLs exist

D Counts each NULL as a separate distinct value

DISTINCT treats all NULL values as the same, so they count as one distinct value.

When using GROUP BY on a column with NULL values, what happens to those NULLs?

A Each NULL is treated as a separate group

B They are excluded from the result

C They are grouped together as one group

D The query fails with an error

GROUP BY groups all rows with NULL in that column into a single group.

By default, where do NULL values appear when sorting with ORDER BY ascending?

A At the end of the result set

B In the middle of the result set

C At the beginning of the result set

D They are removed from the result set

Most SQL databases place NULLs last when sorting ascending, but this can vary.

Does GROUP BY consider NULL values equal or different?

A Depends on the database version

B Different, each NULL is unique

C NULLs are ignored

D Equal, all NULLs form one group

GROUP BY treats all NULLs as equal and groups them together.

If a column has multiple NULLs, how many distinct values does DISTINCT count for those NULLs?

A Zero

B One

C The number of NULLs

D It depends on the query

DISTINCT counts all NULLs as one distinct value.

Explain how NULL values are treated in DISTINCT, GROUP BY, and ORDER BY clauses in SQL.

Think about how SQL handles unknown or missing values in these clauses.

Describe the impact of NULL values on the number of groups or distinct rows returned by SQL queries using DISTINCT or GROUP BY.

Consider how NULLs are treated as equal or different.

Practice

1. What happens to NULL values when using DISTINCT in SQL?

easy

A. NULL values are ignored and not shown in the result.

B. All NULL values are treated as one group and appear once.

C. Each NULL is treated as a unique value and appears multiple times.

D. DISTINCT causes an error if NULL values exist.

Solution

1. Step 1: Understand DISTINCT behavior with NULL

   In SQL, DISTINCT treats all NULL values as equal, grouping them together.

2. Step 2: Result of DISTINCT on NULL values

   This means only one NULL appears in the output, no matter how many NULLs exist.

3. Final Answer:

   All NULL values are treated as one group and appear once. -> Option B

4. Quick Check:

   NULLs grouped once = A [OK]

Hint: NULLs count as one in DISTINCT and GROUP BY [OK]

Common Mistakes:

• Thinking NULLs are ignored in DISTINCT
• Assuming each NULL is unique
• Believing DISTINCT errors on NULL

2. Which of the following is the correct syntax to select distinct values including NULLs from a table named employees on column department?

easy

A. SELECT department DISTINCT FROM employees;

B. SELECT UNIQUE department FROM employees;

C. SELECT DISTINCT(department) FROM employees;

D. SELECT DISTINCT department FROM employees;

Solution

1. Step 1: Identify correct DISTINCT syntax

   The correct SQL syntax to get unique values is SELECT DISTINCT column FROM table;. Parentheses are not used with DISTINCT.

2. Step 2: Check each option

   SELECT DISTINCT department FROM employees; uses correct syntax. SELECT UNIQUE department FROM employees; uses UNIQUE which is not standard SQL. SELECT DISTINCT(department) FROM employees; incorrectly uses parentheses. SELECT department DISTINCT FROM employees; has wrong keyword order.

3. Final Answer:

   SELECT DISTINCT department FROM employees; -> Option D

4. Quick Check:

   Standard DISTINCT syntax = D [OK]

Hint: Use DISTINCT without parentheses for unique values [OK]

Common Mistakes:

• Using UNIQUE instead of DISTINCT
• Adding parentheses after DISTINCT
• Misplacing DISTINCT keyword

3. Given the table sales with column region containing values: ('East', 'West', NULL, 'East', NULL), what will be the result of this query?

SELECT region FROM sales GROUP BY region ORDER BY region ASC;

medium

A. [NULL, 'East', 'West']

B. ['East', 'West', NULL]

C. ['East', NULL, 'West']

D. [NULL, 'West', 'East']

Solution

1. Step 1: GROUP BY groups NULLs together

   The GROUP BY groups all NULL values as one group. So NULL appears once.

2. Step 2: ORDER BY ascending places NULL first

   In ascending order, NULL values come before any text values. Then 'East' and 'West' follow alphabetically.

3. Final Answer:

   [NULL, 'East', 'West'] -> Option A

4. Quick Check:

   NULL first in ORDER BY ASC = A [OK]

Hint: ORDER BY ASC puts NULL before text values [OK]

Common Mistakes:

• Assuming NULL sorts last
• Treating NULL as a string value
• Expecting multiple NULL rows in GROUP BY

4. What is wrong with this query if the goal is to list unique category values including NULLs?

SELECT category FROM products GROUP BY category ORDER BY category;

medium

A. Query is correct and will work as intended.

B. ORDER BY cannot be used with GROUP BY.

C. Missing closing semicolon causes syntax error.

D. GROUP BY ignores NULL values, so NULLs won't appear.

Solution

1. Step 1: Check syntax and logic

   The query groups by category and orders by category. This is valid SQL syntax.

2. Step 2: Understand NULL handling

   GROUP BY includes NULL as a group, so NULL values appear once. ORDER BY sorts NULLs first by default.

3. Final Answer:

   Query is correct and will work as intended. -> Option A

4. Quick Check:

   GROUP BY with ORDER BY and NULLs works = C [OK]

Hint: GROUP BY includes NULLs; ORDER BY works with GROUP BY [OK]

Common Mistakes:

• Thinking NULLs are excluded by GROUP BY
• Believing ORDER BY can't follow GROUP BY
• Assuming missing semicolon breaks query

5. You have a table orders with a column status that can be 'Pending', 'Shipped', or NULL. You want to count how many orders exist for each status, including NULLs as a separate group. Which query correctly achieves this?

hard

A. SELECT status, COUNT(*) FROM orders GROUP BY status HAVING status IS NOT NULL ORDER BY status;

B. SELECT status, COUNT(*) FROM orders WHERE status IS NOT NULL GROUP BY status ORDER BY status;

C. SELECT status, COUNT(*) FROM orders GROUP BY status ORDER BY status ASC;

D. SELECT DISTINCT status, COUNT(*) FROM orders ORDER BY status;

Solution

1. Step 1: Understand grouping with NULL

   GROUP BY status groups NULL values as one group, so counting includes NULLs separately.

2. Step 2: Analyze each option

   SELECT status, COUNT(*) FROM orders GROUP BY status ORDER BY status ASC; groups all statuses including NULL and orders them ascending. SELECT status, COUNT(*) FROM orders GROUP BY status HAVING status IS NOT NULL ORDER BY status; excludes NULL with HAVING. SELECT status, COUNT(*) FROM orders WHERE status IS NOT NULL GROUP BY status ORDER BY status; filters NULL out in WHERE. SELECT DISTINCT status, COUNT(*) FROM orders ORDER BY status; uses DISTINCT with COUNT(*) incorrectly.

3. Final Answer:

   SELECT status, COUNT(*) FROM orders GROUP BY status ORDER BY status ASC; -> Option C

4. Quick Check:

   GROUP BY includes NULL groups = B [OK]

Hint: Use GROUP BY without WHERE to include NULL counts [OK]

Common Mistakes:

• Filtering NULLs out before grouping
• Using HAVING to exclude NULLs unintentionally
• Misusing DISTINCT with aggregate functions