mysql - What does this sql mean with not exists? -


a table this

int char int  int id  name a_id b_id  select count(*) tbl t1 b_id = 12 , not exists(select * tbl t2 t2.a_id = t1.b_id , t2.b_id = t1.a_id)

i think @ least equals 

select count(*) tbl t1 b_id = 12 , not exists(select * tbl t2 t2.a_id = 12 , t2.b_id = t1.a_id)

then mean?

for example, select a_id tbl t1 b_id = 12 gives 1,2,3,4

then following:

select * tbl t2 t2.a_id = 12 , t2.b_id = 1 # null select * tbl t2 t2.a_id = 12 , t2.b_id = 2 # exists select * tbl t2 t2.a_id = 12 , t2.b_id = 3 # exists select * tbl t2 t2.a_id = 12 , t2.b_id = 4 # null

so count(*) 2?

this 1 interesting because finds "symmetries" in a_id , b_id number 12,so counts number of rows in table b_id=12 except ones have symmetric pair, this:

| a_id | b_id | --------------- |  12  |  12  | |  12  |  2   | |  2   |  12  |

or more speaking, query skips counting rows have a_id=x b_id=12 , there exists row a_id=12 , b_12=x.

so not exists there skip counting rows have symmetric pair, don't know can useful in applications interesting nonetheless.


Comments

Post a Comment

Popular posts from this blog

Is there a better way to structure post methods in Class Based Views -

i'm using class based views in django , never specify url in ajax calls , omit action parameter in form because know these requests go through post method of class based view. notice view - profile page make many requests depending on user updating on page - post method become convoluted , if/else mess example: def post(self, request, *args, **kwargs): # handle user changing available date if request.post.get('availabledate') != none: self.updateavailabledate(request) return jsonresponse({'result':'success'}) if request.post.get('newprojectname') != none: creative_user = creativeuserprofile.objects.get(id = request.user.id) project = project.create_project(creative_user, request.post['newprojectname']) project.save() return jsonresponse({'projectid': project.id }) if request.post.get('projectimage') != none , request.files['file']: proj
Read more

performance - Why is XCHG reg, reg a 3 micro-op instruction on modern Intel architectures? -

i'm doing micro-optimization on performance critical part of code , came across sequence of instructions (in at&t syntax): add %rax, %rbx mov %rdx, %rax mov %rbx, %rdx i thought had use case xchg allow me shave instruction , write: add %rbx, %rax xchg %rax, %rdx however, dimay found agner fog's instruction tables , xchg 3 micro-op instruction 2 cycle latency on sandy bridge, ivy bridge, broadwell, haswell , skylake. 3 whole micro-ops , 2 cycles of latency! 3 micro-ops throws off 4-1-1-1 cadence , 2 cycle latency makes worse original in best case since last 2 instructions in original might execute in parallel. now... cpu might breaking instruction micro-ops equivalent to: mov %rax, %tmp mov %rdx, %rax mov %tmp, %rdx where tmp anonymous internal register , suppose last 2 micro-ops run in parallel latency 2 cycles. given register renaming occurs on these micro-architectures, though, doesn't make sense me done way. why wouldn't register renamer
Read more

jquery - Responsive Navbar with Sub Navbar -

good day, forgive me nav vocabulary not best. if question confusing apologize. i'm working on navigation bar needs centered on page @ 768px , above. items in nav open (or perhaps toggle) subnav lives directly below. these nav should appear centered on page. i have been able work through responsive portion of nav bar, having main items align vertically , sub nav menu of each item roll out below, , gave them indent separation. working well. however, issue not responsive side, behavior @ width greater 768px. happens toggling menu's sub navigation pushes other main items down page below sub nav. here's code: $(document).ready(function () { //stick in fixed 100% height behind navbar don't wrap $('#slide-nav.navbar-inverse').after($('<div class="inverse" id="navbar-height-col"></div>')); $('#slide-nav.navbar-default').after($('<div id="navbar-height-col"></d
Read more