It’s a logical DAX function that used to returns the first expression that does not evaluate to BLANK. If all expressions evaluate to BLANK, BLANK is returned.
Syntax
COALESCE(<expression>, <expression>…)
Parameters
expression – Any DAX expression that returns a scalar expression.
Example -1:
Add one measure to understand the working of Coalesce DAX-
Coalesce_DAX = COALESCE( BLANK() , "No value")
As you can see in above DAX, first expression value is blank, so it will return second expression value. See the output in below screen shot.

Coalesce DAX Result -1
Example-2
Add another measure and write below DAX code-
Coalesce_DAX = COALESCE( 12345 , "No value")
Here first expression value is not blank so it will return first expression value, see the below output-

Coalesce DAX Result -2
Example-3
Add one more measure and write below DAX code-
Coalesce_DAX = COALESCE( BLANK() , BLANK())
Here, both expression values are blank, so it will return blank. See the output-

Coalesce DAX Result -3
Refer more DAX functions: DAX functions
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