EF Learning Note 05----------dbcontext Basic Query

Traverse all Entities

// Traverse all students DBSet using (varnew  entities ()) {    foreach (var in db). Student)    {        objectdumper.write (Student. studentname);    }};

Description: Print the names of all students

Query by condition

using(vardb =Newentities ()) {    //Find all students born after January 1, 1991     var  day = Convert.todatetime ("1991-1-1"); varStudents = fromVinchDb. StudentwhereV.birthday >=   DaySelectv; foreach(varStudentinchstudents) {Objectdumper.write (student). Studentname+"-"+student.    Birthday); }};

Execution Result:

Note here: When using LINQ and Lamba , variables must be dumped in advance, and C # functions cannot be used inside LINQ.

The wrong wording:

using(vardb =Newentities ()) {    varStudents = fromVinchDb. StudentwhereV.birthday >=convert.todatetime ("1991-1-1") Selectv; foreach(varStudentinchstudents) {Objectdumper.write (student). Studentname+"-"+student.    Birthday); }};

This writes directly to the error:

Sort in ascending order by birthday (default)

using (varnew  entities ()) {    var from in db. Student                      v.birthday                    Select  v    ; foreach (var in students)    {        objectdumper.write (student);    }};

Descending:descending

var  from inch db. Student                Descending                select v;

Querying a single Entity find method

// find queries a single entity using (varnew  entities ()) {    var student = db. Student.find (1);    Objectdumper.write (student);};

Description: Queries student information based on primary key and returns NULL if not found

Querying individual Entities single method

// single query for individual entities using (varnew  entities ()) {    var from in db. Student                    where v.studentname==" Zhang San "                    Select v;     // Single method can not find records or multiple records will be error     objectdumper.write (student. single ());};

Note: Calling the single method can not find records or multiple records will be error

Querying a single Entity Singleordefault method

 // singleordefault queries a single entity  using  (var  db = new   entities ()) { var  student = from  V in   db. Student  where  v.studentname = = "   John Doe  "  select   V;  // singleordefault method does not find records that return null. If a query to multiple records results in an error   Objectdumper.write (student. Singleordefault ());};  

Description: Calling the Singleordefault method does not find the record put back null if the query to multiple records error

Full case:

//Traverse All Students            using(vardb =Newentities ()) {                varStudents = fromVinchDb. StudentSelectv; foreach(varStudentinchstudents)                {objectdumper.write (student);            }            }; Console.WriteLine ("========== by birthday sort ==========="); //Sort by Birthday ascending            using(vardb =Newentities ()) {                varStudents = fromVinchdb. Student byV.birthday DescendingSelectv; foreach(varStudentinchstudents)                {objectdumper.write (student);            }            }; Console.WriteLine ("==========find querying a single entity ==========="); //find queries a single entity            using(vardb =Newentities ()) {                varStudent = db. Student.find (1);            Objectdumper.write (student);            }; Console.WriteLine ("==========single querying a single entity ==========="); //single query for individual entities            using(vardb =Newentities ()) {                varStudent = fromVinchdb. Studentwherev.studentname=="Zhang San"                                Selectv; //Single method can not find records or multiple records will be errorObjectdumper.write (student.            single ());            }; Console.WriteLine ("==========singleordefault querying a single entity ==========="); //Singleordefault querying a single entity            using(vardb =Newentities ()) {                varStudent = fromVinchdb. StudentwhereV.studentname = ="John Doe"                                Selectv; //Singleordefault method does not find records return NULL, if query to multiple records will be errorObjectdumper.write (student.            Singleordefault ()); };

EF Learning Note 05----------dbcontext Basic Query