SQL specifies functions to map NULL values to and from non-NULL values. COALESCE returns the first passed non-NULL value:
SELECT COALESCE(NULL, 0);
coalesce
----------
0
SELECT COALESCE(NULL, 'I am null.');
coalesce
------------
I am null.
INSERT INTO nullmaptest VALUES ('f'), ('g'), (NULL);
SELECT x, COALESCE(x, 'n/a') FROM nullmaptest;
x | coalesce
--------+----------
f | f
g | g
(null) | n/a
SELECT 'a' || COALESCE(NULL, '') || 'b';
?column?
----------
ab
SELECT SUM(x), COALESCE(SUM(x), 0) FROM aggtest;
sum | coalesce
--------+----------
(null) | 0
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Mapping NULLs to Strings (Part 9/11)
