PostgreSQL：`row_alias is null`和`row_alias is not null`返回值不一致

Question

我在 SQL select 中偶然发现了一个奇怪的现象：

select 1
, ui as row_value
, ui is null as "ui is null"
, ui is not null as "ui is not null"
from user_info ui
where ui.user_id = 1003;

此 SQL 查询的输出： | ？柱子？ |用户界面 |用户界面为空 | ui 不为空 | | :--- | :--- | :--- | :--- | | 1 | ("0000-00-00 00:00:00.000000+00",系统,"0000-00-00 00:00:00.000000+00",系统,0000,正常,00000FEE,f00d0000-0b0b-000a-0e00-0a0e00000b00 ,t,用户名1,用户名2,"用户名1",f,t,f,aad,用户名1,用户名2,,"用户名1 用户名2") |假 |假|

问题在于，

ui is null

和

ui is not null

列的结果行值都是

false

，而

row_value

列包含行本身（不是

null

）。

---

奇怪的是，我无法使用内联值重现此内容，例如：

-- working as expected
select row(1, 2, 'Hello') as original_row
, row(1, 2, 'Hello') is null as row_is_null
, row(1, 2, 'Hello') is not null as row_is_not_null

甚至不使用虚拟表：

-- working as expected

create table table_1
(
    table_1_id int not null primary key generated always as identity,
    title      text
);

create table table_2
(
    table_2_id int not null primary key generated always as identity,
    title      text,
    table_1_id int references table_1 on delete cascade
);

create table table_3
(
    table_3_id int not null primary key generated always as identity,
    title      text,
    table_2_id int references table_2 on delete cascade
);

insert into table_1 (title)
values ('Table1 Row 1')
         , ('Table1 Row 2');

insert into table_2 (title, table_1_id)
values ('Table2 Row 1', 1)
         , ('Table2 Row 2', 2);

insert into table_3 (title, table_2_id)
values ('Table3 Row 1', 1)
         , ('Table3 Row 2', 2);

select t1.title
         , t2
         -- , t2 is null
         -- , t2 is not null
         , t3
         , t3 is null
         , t3 is not null
from table_1 t1
    left join public.table_2 t2 using (table_1_id)
    left join public.table_3 t3 on t2.table_2_id = t3.table_2_id and t3.title = 'Table3 Row 1'
where t1.table_1_id = 1
;

两个最新的代码片段都按预期工作，即

x is null

返回

x is not null

的相反值。

提前致谢

---

编辑：

当使用特定字段的语法时（如

ui.user_id is (not) null

），问题就消失了。那么我想问一下这种形式是否是普遍首选，或者我原来的方法也应该有效？ （我过去直接检查row没有遇到这样的问题）

Answer 1

参见简短示例

create table test (id int,val int);
insert into test values
 (1,100)
,(1,null)
;
select id,val
  ,test is null as is_null
  ,test is not null as is_not_null
from test

输出是

id	瓦尔	is_null	is_not_null
1	100	f	t
1	空	f	f