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问题 Edit: This issue no longer exists in Scala 2.12.6 Original question (for Scala 2.11.7): Why so strange warning? scala> null.asInstanceOf[Double] res0: Double = 0.0 scala> null.asInstanceOf[Double] == null <console>:11: warning: comparing values of types Double and Null using `==' will always yield !!!!false!!!! null.asInstanceOf[Double] == null ^ res1: Boolean = true //!!!! scala> 0.0 == null <console>:11: warning: comparing values of types Double and Null using `==' will always yield false 0

问题 I am new to PDO and I was wondering if there was an equivalent to a mysql statement that checks if a parameter is not null such as: SELECT * FROM table WHERE param IS NOT NULL I tried this: $pdo->prepare('SELECT * FROM ' . $tablename . ' WHERE ' . $field . ' = :' . $field . 'AND param IS NOT NULL'); without any success. I also looked on the web, but did not find anything relevan, can anyone help please ? 回答1: You can use any query in pdo that you could use in mysql directly but your way of

问题 This is a follow-up question to .Net C# String.Join how to output “null” instead of empty string if element value is null? where the answer suggested to use the ?? operator to define a custom null value, but the replacement never got triggered. DataSet myDataSet = new DataSet(); mySqlDataAdapter.Fill(myDataSet); DataTable rotationData = myDataSet.Tables["Table"]; rotationValues = string.Join(", ", from r in rotationData.Rows.OfType<DataRow>() select r[5] ?? "null"); When I change the code to:

问题 Can null be casted to any type? i.e. will the following code work public <T> T foo(Object object){ return (T) object; } Duck duck = foo(new Duck()); // this works Duck duck2 = foo(null); // should this work? Cat cat = foo(null); // should this work? // if they are both null, should this be true? boolean equality = duck2.equals(cat); My question is, is null 'castable to anything'? 回答1: From Javadocs: There is also a special null type, the type of the expression null, which has no name. Because

问题 I noticed that my application was automatically setting all my processes to having been completed as soon as they were created. I looked through it, couldn't find why the date was being updated without being told to, but I found that one of my views was looking for a field that didn't exist. I created the 'complete' field, set it as a not null boolean and tried again. I was getting it being set automatically to true again so I tried setting it as false explicitly in the create method, this

问题 I just found out that I can compare null with an Object like this, if(null != Object) Rather than comparing Object with null , like Object != null What may go wrong if use the former approach? Is that legal? If not then why does compiler accept it? 回答1: There's one thing wrong about it - readability. If you want to write a clean code, you should care about the way it will be read in the future. It needs to be obvious, what it does and why it does a certain thing. If you place the "Object" to

问题 I am doing an outer join of 2 tables on 2 columns. The join should happen if table1.column1=table2.column1 and table1.column2=table2.column2. Since column2 is allowed to be contain null, the join fails whenever the value is null, since null is not equal to null (only a computer scientist could love that). The workaround I came up with is: select table1.column1,table1.colunn1,table2.column1,table2.column2 from table1 left join table2 on table1.column1=table2.column1 and if(table1.column2 is

问题 In Sybase and MSSqlServer TransactSQL, we have a function IsNull(columnName,valueForNull) to return a typed default value when a column value is null. How can I replicate this functionality in SQLite? Example in TransactSQL: select IsNull(MyColumn,-1) as MyColumn from MyTable If MyColumn is null, this expression will return -1 as the value of MyColumn. Want to do something similar in SQLite. (Yes, I know I can write my own wrapper function to handle this - that's not the answer I'm looking

问题 I am writing a library in F#, with some interfaces and base classes that are publicly visible. Generally, I avoid specifying [<AllowNullLiteral>] on my custom types as this complicates the validation logic in my F# code (see this nice post for goods and bads of null handing in F# to get a picture ), and also, F# does not initially allow null for F# types. So, I validate for nulls only for types that accept the null value as valid. However, an issues arises when my library is used from another