Coalesce returns the first none null argument from a set of arguments. Only the first non null argument is return, all subsequent arguments are ignored. The function will evaluate to null if all arguments are null.
PGSQL> SELECT COALESCE(NULL, NULL, 'HELLO WORLD');
coalesce
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'HELLO WORLD'
PGSQL> SELECT COALESCE(NULL, NULL, 'first non null', null, null, 'second non null');
coalesce
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'first non null'
PGSQL> SELECT COALESCE(NULL, NULL, NULL);
coalesce
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