mysql IFNULL() NULLIF() ISNULL()区别

SELECT IFNULL(null,1);  1 SELECT IFNULL(0,1);  0 SELECT ISNULL(null);  1 SELECT ISNULL(0);  0 SELECT NULLIF(null,1); null SELECT NULLIF(1,0); 1 SELECT NULLIF(2,3); 2 SELECT NULLIF(NULL,NULL); null SEL

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