is [not] distinct from 的 null 比较

--在postgresql中经常会比较两个值相同或者不同，但如果比较表达中有一个为null则会比较头痛，如下： postgres=# select null <> null,null = null,null is null,null is not null; ?column? | ?column? | ?column? | ?column? ----------+----------+----------+---------- | | t | f --创建测试数据 postgres=# create table t (id int,name varchar(20)); CREATE TABLE postgres=# insert into t values(1,'rudy'); INSERT 0 1 postgres=# insert into t values(2); INSERT 0 1 postgres=# select * from t; id | name ----+------ 1 | rudy 2 | (2 rows) --expression如果为null与任何值比较，都为null postgres=# select * from t where null <> name; id | name ----+------ (0 rows) --is distinct from 对与非null其意为<> postgres=# select * from t where 'rudy' is distinct from name; id | name ----+------ 2 | --is distinct from 对于expression为null,其会让与null的对比为值，非null对比其本身为假，is distinct from本身比较其是否不同 postgres=# select * from t where null is distinct from name; id | name ----+------ 1 | rudy (1 row) --is not distinct from 对与非null其意为= postgres=# select * from t where 'rudy' is not distinct from name; id | name ----+------ 1 | rudy --is distinct from 对于expression为null,其会让与null的对比为值，非null对比其本身为假，is distinct from本身比较其是相同 postgres=# select * from t where null is not distinct from name; id | name ----+------ 2 | --注意 is [not] distinct from 效率上不如<>,=所以尽量要少用

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