null值不能用=, <>, in, not in作判断，只能使用IS NULL, IS NOT NULL
zlytest=# select 'true' where 3 in (1, 2, 3, null);
?column?
----------
true
(1 row)
zlytest=# select 'true' where 3 not in (1, 2, null);
?column?
----------
(0 rows)
zlytest=# select 'true' where 3 = null;
?column?
----------
(0 rows)
zlytest=# select 'true' where 3 <> null;
?column?
----------
(0 rows)
zlytest=# select 'true' where 3 IS NULL;
?column?
----------
(0 rows)
zlytest=# select 'true' where 3 IS NOT NULL;
?column?
----------
true
(1 row)
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