Adding entries from multiple MySQL tables using one single SQL join statement, but only if there are entries available in the second table -


using 1 single sql query join: how can add entries second table if there corresponding entry available?

project                                source  description   |  source                source_id   |  value ----------------------------           -------------------------------- project 1     |  1                     1           |  additional info 1 project 2     |  null

when type

select project.description, source.value project, source project.source = source.source_id , project.description = "project 1";

as desired receive 

project 1   |   additional info 1

however when replace project 1 project 2 in last line, won't result, because project.source null. possible use single sql query outputs this? 

project 2   |   null

i´m looking query covers both cases.

any ideas?

you can use left join on project table make sure all projects appear in result set if have no matching value in source table. projects project table not match have null value.

select project.description description, source.value value project left join source on project.source = source.source_id

output:

+--------------+--------------------+ | description  |       value        | ---------------+--------------------+ |  project 1   |  additional info 1 | |  project 2   |  null              | +--------------+--------------------+

Comments

Post a Comment

Popular posts from this blog

javascript - Google App Script ContentService downloadAsFile not working -

i have web app developed using google app script htmlservice , html form, populating excel sheet in google drive using spreadsheetapp . , 1 section calling contentservice download data excel file. function doget(e) { // read excel sheet //getappfile(); // render application html template return htmlservice.createtemplatefromfile('index').evaluate() .settitle('go smart') .setsandboxmode(htmlservice.sandboxmode.iframe); } function downloaddoublequatecsvfile() { var sheetid = propertiesservice.getscriptproperties().getproperty('sheetid'); var ss = spreadsheetapp.openbyid(sheetid).getactivesheet(); //var ss = spreadsheetapp.getactivespreadsheet().getactivesheet(); var maxcolumn = ss.getlastcolumn(); var maxrow = ss.getlastrow(); var data = ss.getrange(1, 1, maxrow, maxcolumn).getvalues(); if (data.length > 1) { var csv = ""; (var row = 0; row < data.length; row++) { (var c...
Read more

javascript - Function overwritting -

is there way overwrite function or event handling in function? using addeventlistener, can have many event handlers on element want var test=document.getelementbyid("div"); test.addeventlistener("click",function(){alert("im first");},false); test.addeventlistener("click",function(){alert("im second");},false); // alerts "im first" , after "im second" but if want overwrite function/event example based on client width example this function one() { alert("first"); } function two() { alert("second"); } window.onresize = function() { var test = document.getelementbyid("div"); if (window.innerwidth > 500) { test.onclick = 1 } else { test.onclick = two; } } is possible in javascript? in case use effective little known approach. addeventlistener can accept object property handleevent event handler. in case it's easy overwr...
Read more

c# - Exception when attempting to modify Dictionary -

i creating dictionary manage threads , ran exception when attempt clear dead threads after adding threads dictionary. using system; using system.collections.generic; using system.threading; namespace syscat { public class threadmanager { public static readonly threadmanager globalthreadmanager = new threadmanager( ); /// <summary> /// initializes new instance of <see cref="syscat.threadmanager"/> class. /// </summary> public threadmanager( ) { } /// <summary> /// create new threadmanager threads within basetm , deinitializes basetm /// </summary> public threadmanager( threadmanager basetm ) { this.threads = basetm.threads; basetm.threads.clear( ); } private dictionary<guid,thread> threads = new dictionary<guid, thread>( ); /// <summary> /// attempts obtain unused threadid within ...
Read more