COALESCE for NULL handling in SQL - Step-by-Step Execution

Concept Flow - COALESCE for NULL handling

Start with list of expressions

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Check first expression

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Return it

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Repeat until non-NULL found or list ends

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If all NULL, return NULL

COALESCE checks each expression in order and returns the first one that is not NULL. If all are NULL, it returns NULL.

Execution Sample

SQL

SELECT COALESCE(NULL, NULL, 'apple', 'banana');

Returns the first non-NULL value from the list, which is 'apple'.

Execution Table

Step	Expression Checked	Is NULL?	Action	Value Returned
1	NULL	Yes	Check next expression	NULL
2	NULL	Yes	Check next expression	NULL
3	'apple'	No	Return this value	'apple'
4	'banana'	Not checked	Stop checking	'apple'

💡 Stopped at step 3 because 'apple' is the first non-NULL value.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Current Expression	NULL	NULL	'apple'	'apple'	'apple'
Value Returned	NULL	NULL	NULL	'apple'	'apple'

Key Moments - 2 Insights

Why does COALESCE return 'apple' and not 'banana'?

Because COALESCE returns the first non-NULL value it finds. According to the execution_table, at step 3 it finds 'apple' which is not NULL, so it stops and returns it without checking 'banana'.

What happens if all expressions are NULL?

COALESCE will check all expressions and find all are NULL, so it returns NULL. This is shown by the logic in the concept_flow where if no non-NULL is found, NULL is returned.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value returned after step 2?

A 'apple'

B NULL

C 'banana'

D Error

💡 Hint

Check the 'Value Returned' column at step 2 in the execution_table.

At which step does COALESCE stop checking expressions?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column in the execution_table to see when it returns a value.

If the first expression was 'orange' instead of NULL, what would COALESCE return?

A 'orange'

B 'banana'

C 'apple'

D NULL

💡 Hint

COALESCE returns the first non-NULL expression, so check the first expression value.

Concept Snapshot

COALESCE(expression1, expression2, ...)
Returns the first non-NULL expression in the list.
If all expressions are NULL, returns NULL.
Useful for replacing NULLs with default values.
Evaluates expressions left to right and stops at first non-NULL.

Full Transcript

COALESCE is a SQL function that helps handle NULL values by returning the first non-NULL value from a list of expressions. It checks each expression in order. If the expression is NULL, it moves to the next one. When it finds a non-NULL value, it returns that immediately and stops checking further. If all expressions are NULL, COALESCE returns NULL. For example, COALESCE(NULL, NULL, 'apple', 'banana') returns 'apple' because it is the first non-NULL value. This function is useful when you want to provide a fallback or default value instead of NULL in your query results.