mysql - http://www.sql-ex.ru exercise #39 returns right result but there is a mistake in the query -


were mistake in solution?

the task is
define ships "survived future battles"; being damaged in 1 battle, took part in another.

description of db
database of naval ships took part in world war ii under consideration. database has following relations: classes(class, type, country, numguns, bore, displacement) ships(name, class, launched) battles(name, date) outcomes(ship, battle, result) ships in classes arranged single project. class assigned name of first ship in class under consideration (head ship); otherwise, class name not coincide ship name in database. classes relation includes class name, type (bb battle ship, or bc battle cruiser), country ship built, number of main guns, gun caliber (diameter of gun barrel, in inches), , displacement (weight in tons). ships relation includes ship name, class name, , launch year. battles relation covers name , date of battle ships participated; while result of participation in battle (sunk, damaged, or unharmed - ok) in outcomes relation. notes: 1) outcomes relation may include ships not included in ships relation. 2) sunk ship can not participate in battles after that.

my solution: 

select a.ship (select distinct ship, date outcomes join battles on outcomes.battle=battles.name result = 'damaged') join (select distinct ship, date outcomes join battles on outcomes.battle=battles.name result != 'damaged') b  on a.ship = b.ship a.date < b.date

result: query returned correct dataset on first (available) database, returned incorrect dataset on second checking database. * wrong number of records (less 2)

solved!

select distinct kk.ship (select ship,date outcomes oo,battles bb oo.battle=bb.name) tt  inner join  (select ship,date damagedate outcomes oo,battles bb result='damaged' , oo.battle=bb.name) kk  on tt.ship=kk.ship , tt.date>kk.damagedate

Comments

Post a Comment

Popular posts from this blog

c# - Validate object ID from GET to POST -

i have method allows user access comment have made on site. have post method lets them update comment. my normal solution send entire comment model through, let them update it, update on database when post back. involve sending commentid through in hiddenfor , can manipulated. how can verify commentid sent in get method same i'm getting in post , not able alter comment wish? how can verify commentid sent in method same i'm getting in post? basically have validate following things in post - user logged-in user. authenticated users post comments. commentid in post , should valid commentid , should present in database. userid associated logged-in user should same userid associated comment . comment should contain userid column, can check @ time of update. to make sure update happens comment has been sent in get - hold commentid in session , in post action compare commentid value in session .
Read more

node.js - Custom Model Validator SailsJS -

when using custom model validator checks record within model. return within spread doesn't seem end control flow. //post.js model /** * custom validator **/ types: { isuservalid: function(user_id) { var promise = require('bluebird'); promise.all([ user.findone({id: user_id}) ]) .spread(function(user) { console.log(user); if (user === null || user === undefined) { console.log('failed'); return false; }else{ console.log('passed'); return true; } }); } }, my response standard validation failed response. { "error": "e_validation", "status": 400, "summary": "1 attribute invalid", "model": "post", "invalidattributes": { "owner": [ { "rule": "isuservalid", ...
Read more

php - Find a regex to take part of Email -

i have following emails need extract part from: bestellnummer xxx: 1 von xxx, 1er pack ------------- anfang der nachricht ------------- foo bar baz foo bar baz // <<<<< need text here ------------- ende der nachricht ------------- ------------- anfang der nachricht ------------- foo bar baz foo bar baz ------------- ende der nachricht ------------- there 0 unlimited occurences of ------------- anfang der nachricht ------------- ------------- ende der nachricht ------------- and i'm able extract first part regex: $re = "/------------- .*? -------------.?(.*?).?------------- .*? -------------/s"; but, i'm quite new on learning regex, i'm pretty sure there must better regex extract part (foo bar baz foo bar baz) of text between ------------- anfang der nachricht ------------- ------------- ende der nachricht ------------- as can in different languages, i'm using .? to match between hyphens. i...
Read more