COALESCE for NULL handling in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL function COALESCE do?

COALESCE returns the first non-NULL value from a list of expressions. If all values are NULL, it returns NULL.

beginner

How does COALESCE help with NULL values in SQL queries?

It allows you to replace NULL values with a default or alternative value, making results easier to read and process.

beginner

Example: What is the result of COALESCE(NULL, NULL, 'apple', 'banana')?

The result is 'apple' because it is the first non-NULL value in the list.

beginner

Can COALESCE take more than two arguments?

Yes, COALESCE can take two or more arguments and returns the first non-NULL among them.

intermediate

How is COALESCE different from ISNULL or NVL?

COALESCE is standard SQL and can take multiple arguments, while ISNULL (SQL Server) and NVL (Oracle) usually take two arguments and are vendor-specific.

What will COALESCE(NULL, 'Hello', 'World') return?

A 'Hello'

B 'World'

C NULL

D Error

COALESCE returns the first non-NULL value, which is 'Hello'.

If all arguments in COALESCE are NULL, what is the result?

A Empty string

B NULL

C 0

D Error

COALESCE returns NULL if all arguments are NULL.

Which of these is a valid use of COALESCE?

A COALESCE(column1, column2, 'default')

B COALESCE('default')

C COALESCE()

D COALESCE(NULL)

COALESCE requires at least two arguments; option D is valid.

COALESCE is most similar to which function?

A SUM

B COUNT

C ISNULL

D AVG

COALESCE and ISNULL both handle NULL values by returning a replacement.

What does COALESCE(NULL, NULL, NULL) return?

A Error

B 0

C Empty string

D NULL

All arguments are NULL, so COALESCE returns NULL.

Explain how COALESCE helps handle NULL values in SQL queries.

Think about what happens when you have missing data.

Write a simple SQL example using COALESCE to replace NULL with 'N/A'.

Use COALESCE with two arguments.

Practice

1. What does the SQL function COALESCE do when used in a query?

easy

A. Converts all values to NULL

B. Counts the number of NULL values in a column

C. Deletes rows with NULL values

D. Returns the first non-NULL value from the given list of expressions

Solution

1. Step 1: Understand the purpose of COALESCE

   COALESCE checks each value in order and returns the first one that is not NULL.

2. Step 2: Compare with other options

   Counting NULLs, deleting rows, or converting values to NULL are not what COALESCE does.

3. Final Answer:

   Returns the first non-NULL value from the given list of expressions -> Option D

4. Quick Check:

   COALESCE = first non-NULL value [OK]

Hint: COALESCE picks the first value that is not NULL [OK]

Common Mistakes:

• Thinking COALESCE counts NULLs
• Confusing COALESCE with DELETE
• Assuming COALESCE converts values to NULL

2. Which of the following is the correct syntax to use COALESCE in SQL?

easy

A. COALESCE(column1, column2, 'default')

B. COALESCE(column1; column2; 'default')

C. COALESCE(column1 column2 'default')

D. COALESCE[column1, column2, 'default']

Solution

1. Step 1: Recall COALESCE syntax

   COALESCE uses parentheses and commas to separate arguments: COALESCE(value1, value2, ...).

2. Step 2: Identify correct punctuation

   Only COALESCE(column1, column2, 'default') uses commas and parentheses correctly; others use semicolons, spaces, or brackets incorrectly.

3. Final Answer:

   COALESCE(column1, column2, 'default') -> Option A

4. Quick Check:

   Correct syntax uses commas inside parentheses [OK]

Hint: Use commas inside parentheses for COALESCE arguments [OK]

Common Mistakes:

• Using semicolons instead of commas
• Omitting commas between arguments
• Using square brackets instead of parentheses

3. Given the table Employees with columns FirstName, MiddleName, and LastName, what will this query return?

SELECT FirstName, COALESCE(MiddleName, 'N/A') AS Middle, LastName FROM Employees;

Assuming one row has FirstName='John', MiddleName=NULL, LastName='Doe'.

medium

A. John | N/A | Doe

B. John | NULL | Doe

C. John | '' | Doe

D. John | MiddleName | Doe

Solution

1. Step 1: Understand COALESCE behavior on NULL

   COALESCE returns the first non-NULL value. Since MiddleName is NULL, it returns 'N/A'.

2. Step 2: Apply to the given row

   FirstName is 'John', MiddleName replaced by 'N/A', LastName is 'Doe'.

3. Final Answer:

   John | N/A | Doe -> Option A

4. Quick Check:

   NULL replaced by 'N/A' using COALESCE [OK]

Hint: COALESCE replaces NULL with first non-NULL value [OK]

Common Mistakes:

• Expecting NULL instead of replacement
• Confusing empty string with NULL
• Thinking COALESCE returns column name

4. Identify the error in this SQL query:

SELECT COALESCE(FirstName MiddleName, 'Unknown') FROM Employees;

medium

A. COALESCE cannot have string literals

B. Missing comma between FirstName and MiddleName

C. COALESCE requires only one argument

D. Missing FROM clause

Solution

1. Step 1: Check COALESCE argument syntax

   Arguments must be separated by commas. Here, FirstName and MiddleName are missing a comma.

2. Step 2: Verify other parts

   String literals are allowed, multiple arguments allowed, and FROM clause is present.

3. Final Answer:

   Missing comma between FirstName and MiddleName -> Option B

4. Quick Check:

   Arguments need commas in COALESCE [OK]

Hint: Separate COALESCE arguments with commas [OK]

Common Mistakes:

• Forgetting commas between arguments
• Thinking COALESCE only takes one argument
• Ignoring syntax errors in argument list

5. You have a table Orders with columns OrderID, Discount, and SpecialDiscount. Some rows have NULL in both discount columns. You want to write a query that returns OrderID and the first available discount value, or 0 if both are NULL. Which query correctly uses COALESCE to achieve this?

hard

A. SELECT OrderID, COALESCE(Discount, 0) + COALESCE(SpecialDiscount, 0) AS FinalDiscount FROM Orders;

B. SELECT OrderID, COALESCE(Discount + SpecialDiscount, 0) AS FinalDiscount FROM Orders;

C. SELECT OrderID, COALESCE(Discount, SpecialDiscount, 0) AS FinalDiscount FROM Orders;

D. SELECT OrderID, COALESCE(Discount, SpecialDiscount) AS FinalDiscount FROM Orders;

Solution

1. Step 1: Understand the requirement

   Return the first non-NULL discount from Discount or SpecialDiscount, or 0 if both are NULL.

2. Step 2: Analyze each option

   SELECT OrderID, COALESCE(Discount, SpecialDiscount, 0) AS FinalDiscount FROM Orders; uses COALESCE with three arguments correctly. SELECT OrderID, COALESCE(Discount + SpecialDiscount, 0) AS FinalDiscount FROM Orders; sums discounts before COALESCE, which is wrong if both are NULL. SELECT OrderID, COALESCE(Discount, 0) + COALESCE(SpecialDiscount, 0) AS FinalDiscount FROM Orders; sums COALESCE results, which adds discounts instead of picking first non-NULL. SELECT OrderID, COALESCE(Discount, SpecialDiscount) AS FinalDiscount FROM Orders; misses the default 0.

3. Final Answer:

   SELECT OrderID, COALESCE(Discount, SpecialDiscount, 0) AS FinalDiscount FROM Orders; -> Option C

4. Quick Check:

   COALESCE with fallback 0 picks first non-NULL or 0 [OK]

Hint: Use COALESCE with all options and default last [OK]

Common Mistakes:

• Adding discounts instead of picking first non-NULL
• Forgetting default fallback value
• Using COALESCE incorrectly with expressions