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


Comments

Post a Comment

Popular posts from this blog

php - Vagrant up error - Uncaught Reflection Exception: Class DOMDocument does not exist -

i've set bento/ubuntu-16.04 vagrant box using php 5.6, , getting following error quite few times when try , run shell script: "uncaught reflection exception: class domdocument not exist". i've tried adding following top of script (not together), each no success: "sudo apt-get install php5.6-xml", "sudo apt-get install php5-xml", "sudo apt-get install php-dom" i've tried resetting server "sudo service apache2 restart". does have suggestions?
Read more

vue.js - Create hooks for automated testing -

Image
qa guys complained can't automate frontend testing. because our html looks same outside. lazy hack did this <template> <div :role="$options.name"> ... </div> </template> <script> export default { name: 'vmcomponentname' } </script> which takes name script , applies html. in browser get: <div role="vmcomponentname"> ... </div> line :role="$options.name" goes in every component. is there dry er solutions? please share. additional details i'll explain in details looking for. imagine have vmusercreate form creates user. test case can create user . to automatically test without role, have use following selector: .wrapper > .wrapper > .wrapper > .submit-button the test extremely brittle (will break lot). if use roles can use [role="vmusercreateform"] .submit-button selector. many folds less brittle. so basically, i'm ...
Read more

Add new key value to json node in java -

i have json file : { "id":1, "name":"abc", "addressdetails": { "city":"newyork" } } i wanted add 1 more key-value ("pincode" : "414141") node 'addressdetails'. i tried using : objectmapper mapper = new objectmapper(); jsonnode root = mapper.readtree(new file("d://test.json")); objectnode node = mapper.createobjectnode(); node.with("addressdetails").put("pincode", "414141"); but it's not getting added, there way can ? you can try you should modify file path objectmapper mapper = new objectmapper(); objectnode nodes = mapper.readvalue(new file("d:\\test.txt"), objectnode.class); nodes.with("addressdetails").put("pincode", "414141"); mapper.writer().writevalue(new file("d:\\test.txt"), nodes);
Read more