COALESCE

Syntax

COALESCE(value1, value2, ..., valueN)

What it does

COALESCE() walks the arguments left to right and returns the first value that is not NULL.

If every argument is NULL, the result is NULL.

Example

SELECT
  customer_id,
  COALESCE(phone_home, phone_mobile, 'Unknown') AS best_phone
FROM customers;

Common use cases

• fill missing values with a fallback
• normalize nullable fields for reporting
• avoid NULL leakage into aggregate or display logic

Null semantics

This is one of the most important SQL patterns to use carefully:

COALESCE(NULL, NULL, 'fallback') = 'fallback'

Whereas:

NULL = NULL

is never true in standard SQL predicate evaluation.

Safety check before shipping

• confirm the fallback value is appropriate for reporting or business logic
• verify downstream logic treats the fallback as intended
• check for nullable columns that should stay nullable rather than be normalized