null

问题 I'm teaching myself Objective-C 2.0. I see from various code samples, the following two approaches to test if an object has been initialised. If that's not exactly what these tests do, please correct me. Can you please explain the difference between the following: if (!myObject) and if (myObject == nil) 回答1: All objects are set to nil in the alloc method (or to zero for instance variables). Both of your cases checks if the object is equal to nil (is not initialized) and both will work. They

问题 I would like to find the columns in a table that has a null value in it. Is there a system table that have that information? 回答1: To find columns where "null" values are allowed try... select * from dbc.columns where databasename = 'your_db_name' and tablename = 'your_table_name' and Nullable = 'Y' then to identify the specific rows w/ null values, take the "ColumnName" from the previous result set and run queries to identify results... perhaps throw them in a volatile table if you want to

问题 When the user goes to recipe/new it sends a RecipeCommand object, then after the user sets the information and Recipe Ingredients and Directions , RecipeCommand objects gets send to recipe/new/ingredients . I convert it to a recipe object then save it and return it. Then I send an Ingredients Wrapper which has N RecipeCommands (depends the number the user set at Recipe Ingredients) and the RecipeCommand again and return to recipe/recipe_add_ingredients . This leads to recipe/new/{recipeId}

问题 I want to filter and then sort my ember records such that the null records for a column are returned first. I'm not much familiar with ember and have a rails back ground. filteredByData = myLeads.filterBy('status', 'open').filterBy('doNotCall', false).filterBy('phoneValid', true) filtered = filteredByData.sortBy 'last_dialed_at', 'last_name', 'first_name', 'id' Right now, my records are getting ordered according to the id. What i want is to implement the NULLS LAST/FIRST from sql or postgres

问题 I'm trying to write some code for seeding some test data into the Core Data database in an application I'm developing, about Pokémon. My code for seeding is based on this: http://www.andrewcbancroft.com/2015/02/25/using-swift-to-seed-a-core-data-database/ I am having a slight problem with one thing though. I don't seem to be able to put a nil value inside of a tuple. I'm currently trying to seed some Pokémon Moves into the database. A move can have a bunch of different properties, but which

问题 I have table data which looks like this id | keyword | count | date 1 | ipod | 200 | 2009-08-02 2 | ipod | 250 | 2009-09-01 3 | ipod | 150 | 2009-09-04 4 | ipod | NULL | 2009-09-07 Now what I am after is getting the count of the row which has the newest date but has a not null count. In which case row 3 with count 150.) eg SELECT `keyword`, `count` , max( `date` ) FROM `keywords` WHERE keyword = "ipod" AND `count` IS NOT NULL GROUP BY keyword This returned the right date but not the right

问题 I have a problem here with regards to elements on my pages returning null even though I have typed something in the textbox. What causes this? I want to make a simple CRUD app with a dashboard for final year. Here is my view: @model WebApplication1.Models.Category @{ ViewBag.Title = "Create Category"; } <h2>@ViewBag.Title</h2> @using (Html.BeginForm()) { @Html.AntiForgeryToken() <div class="form-horizontal"> <hr /> @Html.ValidationSummary(true, "", new { @class = "text-danger" }) <div class=

问题 I am trying to import a csv file to Cassandra. The csv file has been generated from Postgres and it contains some null values. Cassandra version: [cqlsh 5.0.1 | Cassandra 3.5 | CQL spec 3.4.0 | Native protocol v4] I am using this query to import: copy reports (id,name,user_id,user_name,template_id,gen_epoch,exp_epoch,file_name,format,refile_size,is_sch,job_id,status,status_msg) from '/home/reports.csv' with NULL='' and header=true and DELIMITER = ','; I keep on receiving this error: Failed to

问题 First of all, excuse me for the bad title. I'm still so confused about this behavior, that I wasn't able to describe it; however I was able to reproduce it and broke it down to an (goofy) example. Please, could you be so kind and explain why other.list appears to be full of NULL s after calling lapply() ? some.list <- rep(list(rnorm(1)),33) other.list <- rep(list(), length = 33) lapply(seq_along(some.list), function(i, other.list) { other.list[[i]] <- some.list[[i]] browser() }, other.list) I

问题 I am working on a simple ETL project which reads CSV files, performs some modifications on each column, then writes the result out as JSON. I would like downstream processes which read my results to be confident that my output conforms to an agreed schema, but my problem is that even if I define my input schema with nullable=false for all fields, nulls can sneak in and corrupt my output files, and there seems to be no (performant) way I can make Spark enforce 'not null' for my input fields.