where

问题 public interface ICrudService<T> where T: Entity, new() What is the meaning of " new() " at the end of the above code? 回答1: new() means that T has to have a parameterless constructor. This is a help to enable you to construct objects of type T in your generic class/method: public T Create() { return new T(); } 来源： https://stackoverflow.com/questions/5461963/c-sharp-code-confusion-of-where-clause

问题 I'm using SQL Server 2012 and I would like to know if I write the sentence: SELECT MyDateTimeColumn FROM MyTable WHERE CAST(MyDateTimeColumn AS DATE) = '2014-07-09' is a slower way to trim the time over DATETIME columns, I have searched but I can't find anything about this strict sentence and I don't know how to show that impresive statistics about time consuming cast/convert to probe it myself. 回答1: In SQL 2008+, CAST(foo AS date) is sargable, along with a few other manipulations. Look at

问题 I'm using SQL Server 2012 and I would like to know if I write the sentence: SELECT MyDateTimeColumn FROM MyTable WHERE CAST(MyDateTimeColumn AS DATE) = '2014-07-09' is a slower way to trim the time over DATETIME columns, I have searched but I can't find anything about this strict sentence and I don't know how to show that impresive statistics about time consuming cast/convert to probe it myself. 回答1: In SQL 2008+, CAST(foo AS date) is sargable, along with a few other manipulations. Look at

问题 Not sure how I can do this. Basically I have variables that are populated with a combobox and then passed on to form the filters for a MQSQL query via the where clause. What I need to do is allow the combo box to be left empty by the user and then have that variable ignored in the where clause. Is this possible? i.e., from this code. Assume that the combobox that populates $value1 is left empty, is there any way to have this ignored and only the 2nd filter applied. $query = "SELECT * FROM

问题 This question already has answers here : Closed 8 years ago . Possible Duplicate: SQL: What's the difference between HAVING and WHERE? I have seen various discussions on WHERE and HAVING . I still have a question: is HAVING used only when considering aggregates, or can it be used in more general terms: whenever you have created or aliased a field in your query? I know that WHERE should always be used when possible. 回答1: HAVING specifies a search for something used in the SELECT statement. In

问题 I'm using Parse and I can't make this method to work. The idea is to send push to a single user with an already known user identifier from any platform within 'Cloud code'. Queries I've tried without push being delivered to that user (actually no push was delivered to any user) PFInstallation through the user pointer: var targetUser = new Parse.User(); targetUser.id = userID; var query = new Parse.Query(Parse.Installation); query.equalTo('user', targetUser); Query in PFUser itself var query =

问题 I need to create a query which checks if a field (string) contains one or more words supplied at run time. Basically I need to be able to ask a WhereOr question. This seems like it should be a common issue when dealing with LinqToSql. I found the following reference but can't make sense out of it - and have no idea how to use it in my project. i've tried the following loop: var query = from d in context.Domains select d; for (int i = 0; i < words.Length; i++) { query = query.Where(d => d.Name

问题 How can I use the DISTINCT clause with WHERE? For example: SELECT * FROM table WHERE DISTINCT email; -- email is a column name I want to select all columns from a table with distinct email addresses. 回答1: If you mean all columns whose email is unique: SELECT * FROM table WHERE email in (SELECT email FROM table GROUP BY email HAVING COUNT(email)=1); 回答2: May be by : SELECT DISTINCT email,id FROM table where id='2'; 回答3: select t1.* from YourTable as t1 inner join (select email from YourTable

问题 I need to work with multiple condition using numpy. I'm trying this code that seem to work. My question is: There is another alternative that can do the same job? Mur=np.array([200,246,372])*pq.kN*pq.m Mumax=np.array([1400,600,700])*pq.kN*pq.m Mu=np.array([100,500,2000])*pq.kN*pq.m Acreq=np.where(Mu<Mur,0,"zero") Acreq=np.where(((Mur<Mu)&(Mu<Mumax)),45,Acreq) Acreq=np.where(Mu>Mumax,60,Acreq) Print(Acreq) ['0' '45' '60'] 回答1: Starting with this: Mur = np.array([200,246,372])*3*5 Mumax = np

问题 First Query: Select * from table1 inner join table2 on table1.Id = table2.Id Second Query: Select * from table1, table2 where table1.Id = table2.Id What is difference between these query regarding performance which should one use? 回答1: The two statements you posted are logically identical. There isn't really a practical reason to prefer one over the other, it's largely a matter of personal style and readability. Some people prefer the INNER JOIN syntax and some prefer just to use WHERE.