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问题 So here is my issue I am comparing new and old values when a table row is being updated. But the new or old value will sometimes be null. So the code below doesn't work. Can can I remedy this issue? Thank You BEFORE UPDATE ON mytable FOR EACH ROW BEGIN IF OLD.assignedto != NEW.assignedto THEN INSERT INTO history ( asset , changedfield , oldvalue , newvalue ) VALUES ( NEW.asset, 'assignedto', OLD.assignedto, NEW.assignedto ); END IF; END$$ 回答1: Try: IF ( (OLD.assignedto IS NOT NULL AND NEW

问题 So here is my issue I am comparing new and old values when a table row is being updated. But the new or old value will sometimes be null. So the code below doesn't work. Can can I remedy this issue? Thank You BEFORE UPDATE ON mytable FOR EACH ROW BEGIN IF OLD.assignedto != NEW.assignedto THEN INSERT INTO history ( asset , changedfield , oldvalue , newvalue ) VALUES ( NEW.asset, 'assignedto', OLD.assignedto, NEW.assignedto ); END IF; END$$ 回答1: Try: IF ( (OLD.assignedto IS NOT NULL AND NEW

问题 I discovered a strange behaviour of my program and after futher analysis, I was able to find that there is probably something wrong either in my C# knowledge or somewhere else. I beleive it's my mistake but I cannot find an answer anywhere... public class B { public static implicit operator B(A values) { return null; } } public class A { } public class Program { static void Main(string[] args) { A a = new A(); B b = a ?? new B(); //b = null ... is it wrong that I expect b to be B() ? } } The

问题 I have the following line of code if (DBNull.Value.Equals(o) || o != null) where o is object o in row.ItemArray I keep getting an error of --> Xml type "List of xdt:untypedAtomic" does not support a conversion from Clr type "DBNull" to Clr type "String". What I don't understand is that when I step through my code this if should be catching this and performing my alternate action but it does not? Can someone please shed some light for me. Thank you! 回答1: Try using Convert.IsDBNull method. 回答2:

问题 How can I format the Null Dates in my Oracle SQL to 00/00/0000. I am using NVL function but it doesn't recognize the 00/00/0000 as the date format. Is there any Date Formatting available in Oracle SQL which formats null date to 00/00/0000 回答1: Do the to_char first and wrap it in the NVL. For example, select nvl(to_char(null, 'DD-MM-YYYY'), '00-00-0000') from dual 回答2: Use this function to assign default values of null values of date: SELECT id, NVL(start_date, to_date('20020101','YYYYMMDD'))

问题 Both free(NULL) and ::operator delete(NULL) are allowed. Does the allocator concept (e.g. std::allocator also allow deallocate(NULL,1) , or is it required to put your own guard around it? 回答1: You'll need to add your own check. According to §20.4.​1.1/8, deallocate requires: p shall be a pointer value obtained from allocate(). n shall equal the value passed as the first argument to the invocation of allocate which returned p. allocate throws an exception when storage can't be given (§20.4.​1

问题 I am reading in a JSON file (using Google's GSON ). One of my tests checks program's behavior in event file a given key is missing. JsonElement value = e.getAsJsonObject().get(ENVIRONMENT); My expectation is that when .get(ing) this key, i would get null . Turns out i do. When i .get(ENVIRONMENT) , value returned is null . When i test it, i actually get a " not null ". Weird, considering, GSON's javadoc says " provides check for verifying if this element represents a null value or not " if

问题 i'm working on a android project and i had the need to check for internet connection. i searched the web and i found a solution here on stackoverflow. However, i'm having problems on checking the internet state. I already searched everywhere but i can't find any solution for my problem. Here is the manifest: <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" /> <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" /> <uses-permission android:name=

问题 I know this is a good idea in MySQL. If I recall correctly, in MySQL it allows indexes to work more efficiently. 回答1: It's always a good ideal to keep columns from being NULL if you can avoid it, because the semantics of using are so messy; see What is the deal with NULLs? for good a discussion of how those can get you into trouble. In versions of PostgreSQL up to 8.2, the software didn't know how to do comparisons on the most common type index (the b-tree) in a way that would include finding

问题 I'm working on a table design that could involve many NULL values in about 10 fields maybe 75% of the time the fields would be unused. I just generated some fake data (a million records) and could not sense any impact on SQL Server 2005. Size difference was in the KB. Performance - no measurable difference after adding an index to the 3 non-nullable columns. I know SQL Server 2008 has the sparse columns feature (which I assume is going to be used on the next SharePoint's UserData table). I