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

Concept Flow - COALESCE for NULL handling

Start with list of expressions

↓

Check first expression

↓

Return it

↓

Repeat until non-NULL

↓

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

PostgreSQL

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	Output
1	NULL	Yes	Check next expression	No output yet
2	NULL	Yes	Check next expression	No output yet
3	'apple'	No	Return this value	'apple'
4	'banana'	Not checked	Already returned	'apple'

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

Variable Tracker

Expression	Value	Checked	Returned
1st	NULL	Yes	No
2nd	NULL	Yes	No
3rd	'apple'	Yes	Yes
4th	'banana'	No	No

Key Moments - 2 Insights

Why does COALESCE stop checking after finding 'apple'?

Because COALESCE returns the first non-NULL value it finds, so after step 3 in the execution_table, it stops checking further expressions.

What happens if all expressions are NULL?

COALESCE returns NULL, as shown by the logic in the concept_flow where if all expressions are NULL, the final output is NULL.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at step 2?

A No output yet

B 'apple'

C NULL

D 'banana'

💡 Hint

Check the 'Output' column for step 2 in the execution_table.

At which step does COALESCE return a value?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look for the step where 'Is NULL?' is 'No' and 'Action' is 'Return this value' in the execution_table.

If the third expression was NULL instead of 'apple', what would COALESCE return?

A NULL

B Error

C 'banana'

D 'apple'

💡 Hint

COALESCE returns the first non-NULL value; if third is NULL, it checks the fourth expression.

Concept Snapshot

COALESCE(expr1, expr2, ...)
Returns the first non-NULL expression.
Checks expressions in order.
If all NULL, returns NULL.
Useful for replacing NULL with defaults.

Full Transcript

COALESCE is a function that helps handle NULL values by checking each expression in order and returning the first one that is not NULL. If all expressions are NULL, it returns NULL. For example, COALESCE(NULL, NULL, 'apple', 'banana') returns 'apple' because it is the first non-NULL value. The function stops checking once it finds a non-NULL value. This behavior is useful when you want to provide default values instead of NULL in your database queries.