Further exploring database operations from C#/.NET Jim Warren, COMPSCI 280 S Enterprise Software Development

Today’s learning objectives  To be able to use the LINQ (language-integrate query) approach for database queries in C#/.NET  To be able to update the database from C#/.NET  To conceptualise an application in terms of CRUD (create, read, update, delete) operations  To consider the state of an object instance in relation to the database context COMPSCI 2802

Last lecture…  We learned to connect to a database, set up a model of its tables and create queries like:  This is OK, but it has some disadvantages  We don’t get support from the IDE for the query itself  It’s an arbitrary string – no type checking, no intellisense  It’s a bit of a pain to always need to figure out the type of the returned instances  It’s also a potential security hole… COMPSCI 2803 var x2 = db.Database.SqlQuery ("select * from employee");

SQL injection COMPSCI 2804

The LINQ approach  Makes the query actually a statement in the C# language  SQL’s select-from-where is turned on its head a bit  Makes the intellisense work better to use from-where-select order  In this case the return value is a collection of objects of a class we’ve already defined in our Model  Because we had said: public DbSet Employees { get; set; } and had defined the Employee class with property names exactly aligned to the columns of the DBMS table COMPSCI 2805 using (EmployeesContext db = new EmployeesContext()) { var emps = from e in db.Employees where e.DateOfBirth.Year < 1975 select e; string surname1=emps.First().surname } We get access to all C#’s operators, too!

LINQ Join  We could, of course, have sent join queries to MySQL as a string, but LINQ brings Join into C#  Notice the ‘select’  Even if we included in the model definitions of the ‘categories’ and ‘products’ tables, we’d need a new class to hold the combination of a product name and a category name  The ‘new’ statement is constructing an anonymous type with properties ProductName and Category  innerJoinQuery will be implicitly typed to a generic collection of instances with the generic’s type parameter being this new 2-property anonymous type  We can still do a foreach statement on innerJoinQuery to loop through the results COMPSCI 2806 var innerJoinQuery = from category in db.categories join prod in db.products on category.ID equals prod.CategoryID select new { ProductName = prod.Name, Category = category.Name }; Notice we said ‘equals’ as a keyword (not the = sign); this gives system most flexibility to exploit indices or other optimise the join

More LINQ – group by  Get a count of how many employees with each surname  Use ‘group’… ‘by’ keyword pair  Note use of ‘into’ and naming a second index variable (‘g’ in this case)  Note the special property Key is the thing we grouped by COMPSCI 2807 var cnts = from emp in db.Employees group emp by emp.Surname into g select new {s=g.Key, cnt=g.Count()}; foreach (var c in cnts) Console.WriteLine("Key: " + c.s + " Count: " + c.cnt);

Or method based GroupBy  Use GroupBy method allowing a lambda to specify just what we want the grouping Key to be  And you can apply one method onto the result of another COMPSCI 2808 foreach (var c in db.Employees.GroupBy(a=>a.Surname.Substring(0,1))) Console.WriteLine("Key2: " + c.Key+ " Count2: " + c.Count()); foreach (var c in db.Employees.GroupBy(a => Math.Floor(a.DateOfBirth.Year/10f))) Console.WriteLine("Key2: " + c.Key + "0's Count2: " + c.Count()); foreach (var c in db.Employees.Where(a=>a.Surname.CompareTo("F")>=0).GroupBy(a=>a.Su rname.Substring(0,1))) Console.WriteLine("Key2: " + c.Key+ " Count2: " + c.Count()); Count of employees by first letter of surname Count of employees by decade of birth

What about the rest of the CRUD?!  CRUD: Create, Read, Update, Delete  A CRUD matrix can be a useful specification for the scope of programming tasks  E.g. to describe the ‘life cycle’ of each entity in a system; e.g. a hotel reservation  On some screen (e.g. ‘booking’) it is created (SQL INSERT, not CREATE like making a new table)  There may be a screen to UPDATE it (e.g. ‘change booking’) which probably also reads the current value (‘R’ of CRUD, SQL SELECT)  And there will be one or more ways to DELETE it (e.g. from a cancellation action on the change booking screen or a dedicated cancellation screen, and also once the person has checked in) – then again, you might not actually delete in a SQL sense, but UPDATE it to cancelled or utilised status COMPSCI 2809

CRUD matrix (well, for one entity) Program (or ‘screen’)Entity=reservation Make reservationC Check reservationR Review/change reservationRU Review/cancel reservationRD* Client arrives (takes up reserved room)D** COMPSCI  For a more involved system you may have many entities represented as further columns  Each cell marked with a subset of the letters in ‘CRUD’ * Or update status to cancelled ** Or update status to taken

The UPDATE  Native SQL version  Given that I have a reference to an object emp of class Employee with updated values: COMPSCI using (EmployeesContext db = new EmployeesContext()) { string sql=String.Format( "UPDATE employee SET Surname='{0}',GivenNames='{1}',"+ "DateOfBirth='{2:yyyy-MM-dd}' WHERE id={3}", emp.Surname,emp.GivenNames,emp.DateOfBirth,emp.id); db.Database.ExecuteSqlCommand(sql);... } Here we’re just building the text of a SQL command with the help of String.Format to plug in the parameters from our C# code at the curly braces (note the 3 rd parameter - #2 counting from 0! – is formatted so MySQL recognises the date literal; also note carefully the double quotes interpreted by C# and the signle quotes that are for MySQL Then we just.ExecuteSqlCommand (as compared to doing. SqlQuery) on the database context to tell MySQL to have at it Re-establish the database connection

Native INSERT  Similar to the UPDATE, once we have a database context established we can do INSERT (or DELETE) SQL COMPSCI string sql_ins = String.Format("INSERT INTO employee (GivenNames,SurName,DateOfBirth) "+ "VALUES ('{0}','{1}','{2:yyyy-MM-dd}')","Bob", "Barker",DateTime.Now); db.Database.ExecuteSqlCommand(sql_ins); string sql_del = "delete from employee where GivenNames='Bob'"; db.Database.ExecuteSqlCommand(sql_del);

The UPDATE v2  Entity Framework version  If you’ve gotten an object from the database, the system tracks that that object is “attached” to the database context  If you want to update a record  Retrieve it  Change properties of the object as you wish  Save the changes COMPSCI using (EmployeesContext db = new EmployeesContext()){ var updEmp = from emp in db.Employees where emp.Surname == "Barker" select emp; foreach (Employee e3 in updEmp) e3.Surname = "Good"; db.SaveChanges();... } Every employee with surname ‘Barker’ gets updated to surname ‘Good’

INSERT by adding  If you simply create a ‘new’ object of the class of a record, it isn’t attached to the database context by default  However, you use the appropriate DbSet’s Add method to attach it  Use the database context’s SaveChanges method to make the insertion stick COMPSCI using (EmployeesContext db = new EmployeesContext()) {... var e1 = new Employee{GivenNames="Tommy", Surname="Tank"}; db.Employees.Add(e1); db.SaveChanges(); Adds a new employee with name “Tommy Tank”, letting the ‘id’ be supplied automatically and leaving DateOfBirth unspecified (probably will be set to 1 Jan 1 AD !) Note curly-brace “object initializer” syntax. Invokes constructor and then assigns the specified values to the named properties of the new object

Delete  To delete  Get a handle on the relevant object(s) (e.g. with a query, like we did for update)  Invoke the Remove method of the DbSet with the object to delete as a parameter  Note: if deleting (or in some cases updating) where there are foreign key constraints you may need to think carefully about the ‘cascade’ of effects with respect to referential integrity (see us/library/aa902684(v=sql.80).aspx) COMPSCI using (EmployeesContext db = new EmployeesContext()) {... var deletedEmp = from emp in db.Employees where emp.Surname == "Tank" select emp; foreach (Employee emp in deletedEmp) db.Employees.Remove(emp); db.SaveChanges();

Where we’re up to  We’ve learned some basics of.NET and C#  We’ve seen that we can interact with our DBMS by sending it native SQL or using C#/.NET language-embedded syntax (LINQ and Entity Framework)  Now…  Work the second labsheet (if you haven’t already)  Get seriously into Assignment 2  See you other side of the break!  FYI, I’m on leave first week of the break, but will be at work and answering questions second week of the break COMPSCI 28016