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问题 I have a column called CODE in a MySQL table which can be NULL. Say I have some rows with CODE='C' which I want to ignore in my select result set. I can have either CODE=NULL or CODE!='C' in my result set. The following query does not return a row with CODE as NULL: SELECT * from TABLE where CODE!='C' But this query works as expected and I know it is the right way to do it. SELECT * from TABLE where CODE IS NULL OR CODE!='C' My question is why does having only CODE!='C' does not return rows

问题 Hibernate used with PostgreSQL DB while ordering desc by a column puts null values higher than not null ones. SQL99 standard offers keyword "NULLS LAST" to declare that null values should be put lower than not nulls. Can "NULLS LAST" behaviour be achieved using Hibernate's Criteria API? 回答1: Given that HHH-465 is not fixed and is not going to get fixed in a near future for the reasons given by Steve Ebersole, your best option would be to use the CustomNullsFirstInterceptor attached to the

问题 Here's a context where I have seen that: NSMutableArray *controllers = [[NSMutableArray alloc] init]; for (unsigned i = 0; i < kNumberOfPages; i++) { [controllers addObject:[NSNull null]]; } why not nil in that place? 回答1: Directly from Apple: The NSNull class defines a singleton object you use to represent null values in situations where nil is prohibited as a value (typically in a collection object such as an array or a dictionary). So in your example, that's exactly what's happening, the

问题 I need to check if value is defined as anything, including null. isset treats null values as undefined and returns false . Take the following as an example: $foo = null; if(isset($foo)) // returns false if(isset($bar)) // returns false if(isset($foo) || is_null($foo)) // returns true if(isset($bar) || is_null($bar)) // returns true, raises a notice Note that $bar is undefined. I need to find a condition that satisfies the following: if(something($bar)) // returns false; if(something($foo)) //

问题 We noticed that lots of bugs in our software developed in C# (or Java) cause a NullReferenceException. Is there a reason why "null" has even been included in the language? After all, if there were no "null", I would have no bug, right? In other words, what feature in the language couldn't work without null? 回答1: Anders Hejlsberg, "C# father", just spoke about that point in his Computerworld interview: For example, in the type system we do not have separation between value and reference types

问题 In a mysqli prepared statement, a NULL gets turned into '' (in the case of a string) or 0 (in the case of an integer). I would like to store it as a true NULL. Is there any way of doing this? 回答1: I know this is an old thread, but it's possible to bind a true NULL value to the prepared statements (read this). You can, in fact, use mysqli_bind_parameter to pass a NULL value to the database. simply create a variable and store the NULL value (see the manpage for it) to the variable and bind that

问题 Comparable contract specifies that e.compareTo(null) must throw NullPointerException . From the API: Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false . On the other hand, Comparator API mentions nothing about what needs to happen when comparing null . Consider the following attempt of a generic method that takes a Comparable , and return a Comparator for it that puts null as the minimum element.

问题 I have a dataframe with ~300K rows and ~40 columns. I want to find out if any rows contain null values - and put these 'null'-rows into a separate dataframe so that I could explore them easily. I can create a mask explicitly: mask = False for col in df.columns: mask = mask | df[col].isnull() dfnulls = df[mask] Or I can do something like: df.ix[df.index[(df.T == np.nan).sum() > 1]] Is there a more elegant way of doing it (locating rows with nulls in them)? 回答1: [Updated to adapt to modern

问题 How do I remove the null elements from a list of lists, like below, in R: lll <- list(list(NULL),list(1),list("a")) The object I want would look like: lll <- list(list(1),list("a")) I saw a similar answer here: How can I remove an element from a list? but was not able to extend it from simple lists to a list of lists. EDIT Bad example above on my part. Both answers work on simpler case (above). What if list is like: lll <- list(list(NULL),list(1,2,3),list("a","b","c")) How to get: lll <- list

问题 When using the Java 8 Optional class, there are two ways in which a value can be wrapped in an optional. String foobar = <value or null>; Optional.of(foobar); // May throw NullPointerException Optional.ofNullable(foobar); // Safe from NullPointerException I understand Optional.ofNullable is the only safe way of using Optional , but why does Optional.of exist at all? Why not just use Optional.ofNullable and be on the safe side at all times? 回答1: Your question is based on assumption that the