sql server - Why does UNION returns only one null? -

i understand null represents missing/unknown value, null not equal null because 2 unknown things cannot compared. example

if null = null     select 'nulls equal' else     select 'nulls not equal' 

results in 'nulls not equal' used = instead of is null or is not null here emphasize fact 2 nulls cannot compared.

coming union, union supposed eliminate duplicate values. expecting below code return 2 rows each null since 2 null values not equal, 1 null in result set.

(select null col1) union  (select null col1) 

why sql's interpretation of 'null unknown value' change in above 2 statements?

null not comparable, sql have concept of "is distinct from"
sql server has connect item it

• 1 distinct null = true
• 1 = null false

for completeness, null distinct null = false

i guess distinct , union use is distinct from (as pரதீப் mentioned above)

now, sql server does have is distinct from in intersect , except

declare @t1 table (t1col int); insert @t1 values (1), (null), (2), (3), (3), (5), (5);  declare @t2 table (t2col int); insert @t2 values (1), (null), (3), (4);  select distinct 't1 exists t2', *  @t1 t1 exists (select * @t2 t2 t1.t1col = t2.t2col);  t1 exists t2    1 t1 exists t2    3 t1 exists t2    3  select distinct 't1 intersect t2', * @t1 intersect select 't1 intersect t2', * @t2;  t1 intersect t2 null t1 intersect t2 1 t1 intersect t2 3 

intersect , except remove duplicates because semi-join
exists anti-join btw

for completeness

select 't1 exists t2', *  @t1 t1 not exists (select * @t2 t2 t1.t1col = t2.t2col);  t1 exists t2    null t1 exists t2    2 t1 exists t2    5 t1 exists t2    5  select 't1 except t2', *  @t1 except select 't1 except t2', * @t2;  t1 except t2    2 t1 except t2    5 

example taken answer why except exist in t-sql? added nulls