null | 易学教程

How to check if a BOOL is null?

阅读更多关于 How to check if a BOOL is null?

问题 Is there a way I can check to see if a value is NULL/Nil before assigning it to a BOOL? For example, I have a value in a NSDictionary that can be either TRUE/FALSE/NULL mySTUser.current_user_following = [[results objectForKey:@"current_user_following"]boolValue]; When the value is NULL I get the following error *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSNull boolValue]: unrecognized selector sent to instance I would like to be able to handle

Where Clause Rejecting Rows if NULL occurred

阅读更多关于 Where Clause Rejecting Rows if NULL occurred

问题 A Theory Question... When a set of queries as given below is fired then... Create table Temp1(C1 varchar(2)) Create table Temp2(C1 varchar(2)) insert into Temp1 Values('A'),(NULL),('B') insert into Temp2 Values('B'),(NULL),('C'),(NULL) select *from Temp1 A,Temp2 B where A.C1 <> B.C1 ...gives... I used A.C1 <> B.C1 in the Where clause. But I expect... To get expected result as output I needed to use ISNULL(A.C1,'') <> ISNULL(B.C1,'') in the Where clause. My Question is why do I need to use

How to differentiate between empty string and null with OpenCSV

阅读更多关于 How to differentiate between empty string and null with OpenCSV

问题 This is my code: CSVReader reader = new CSVReader(isReader); while ((col = reader.readNext()) != null) { String c1 = col[1]; } this is my csv file: "a","","c" "1",,"3" Is there a way I can differentiate between null and ""? OpenCSV seems to treat everything as non-null String. Can I tell it to treat empty fields as null? 回答1: It is not possible. There is no difference between an empty string and the java specific null in the CSV format's philosophy. The null is an empty reference in the java

How to differentiate between empty string and null with OpenCSV

阅读更多关于 How to differentiate between empty string and null with OpenCSV

js null,undefined判断

阅读更多关于 js null,undefined判断

var exp = null ; if (! exp ) 如果 exp 为 undefined，或数字零，或 false，也会得到与 null 相同的结果，虽然 null 和二者不一样。注意：要同时判断 null、undefined、数字零、false 时可使用本法。 var exp = null ; if (! exp && typeof exp != "undefined" && exp != 0) { alert ( "is null" ); } typeof exp != "undefined" 排除了 undefined； exp != 0 排除了数字零和 false。更简单的正确的方法： var exp = null ; if ( exp === null ) { alert ( "is null" ); } 尽管如此，我们在 DOM 应用中，一般只需要用 (!exp) 来判断就可以了，因为 DOM 应用中，可能返回 null，可能返回 undefined，如果具体判断 null 还是 undefined 会使程序过于复杂。来源： https://www.cnblogs.com/henw/archive/2011/05/12/2044166.html

Is memory address 0x0 usable?

阅读更多关于 Is memory address 0x0 usable?

问题 I was wondering... what if when you do a new, the address where the reservation starts is 0x0? I guess it is not possible, but why? is the new operator prepared for that? is that part of the first byte not usable? it is always reserved when the OS starts? Thanks! 回答1: The null pointer is not necessarily address 0x0 , so potentially an architecture could choose another address to represent the null pointer and you could get 0x0 from new as a valid address. (I don't think anyone does that, btw,

Using Case Statement in SQL with parameter/variable to check for Null values

阅读更多关于 Using Case Statement in SQL with parameter/variable to check for Null values

问题 I am trying to write a SQL Select statement to return records based on a user input through a front end. I want to write the Select statement like this: SELECT somefields FROM sometable WHERE CASE variable WHEN 'blank' THEN field IS NULL ELSE field = field END Basically I either want to filter a column to find NULL values or ignore the filter and return all values depending on the value of the variable. I know that the results of the CASE statement is not executable but how can I do this? 回答1

Why can't typed optional arguments have a default of Null?

阅读更多关于 Why can't typed optional arguments have a default of Null?

问题 In ActionScript 3, when you declare an optional argument by giving it a default value, the value null cannot be used on typed arguments. function Action(Param:int=null){ // 1184: Incompatible default value of type Null where int is expected. } function Action(Param:int=0){ // No compiler errors } Any workarounds for this, or general purpose values that can apply to all data types? 回答1: You can change your int to Number and then can set it to NaN which is a special number that means 'not a

UNION causes “Conversion failed when converting the varchar value to int”

阅读更多关于 UNION causes “Conversion failed when converting the varchar value to int”

问题 I tried to search for previous articles related to this, but I can't find one specific to my situation. And because I'm brand new to StackOverflow, I can't post pictures so I'll try to describe it. I have two datasets. One is 34 rows, 1 column of all NULL s. The other 13 rows, 1 column of varchar s. When I try to UNION ALL these two together, i get the following error: Conversion failed when converting the varchar value to data type int. I don't understand why I'm getting this error. I've

Difference between MySQL IS NOT NULL and != ''

阅读更多关于 Difference between MySQL IS NOT NULL and != ''

问题 Is there any difference between MySQL IF (myText IS NOT NULL) THEN and IF (myText != '') THEN 回答1: Yes there is a big difference between a NULL value and a blank/empty value. Here's one resource that describes the differences. When myText IS NULL : myText IS NOT NULL evaluates to FALSE myText != '' evaluates to NULL (which essentially behaves the same as FALSE would in this specific case you wrote) However, you should not get into the habit of treating them the same, since most of the time