Order by of the LINQ to SQL statement (5)

Order by Operation

Scenario: Sort the query out of the statement, such as by time and so on.

Description: Sorts the collection by the specified expression; delay: Sorts the collection by the specified expression; delay, which is ascending by default, plus descending for descending, and the corresponding extension method is Order by and OrderByDescending

1. Simple Form

This example uses order-by to sort employees by hire date:

var q =

From E in DB. Employees

E.hiredate

Select E;

Description: The default is ascending

2. With conditional form

Note: The order of Where and order by is not important. In T-SQL, where and order by have strict location restrictions.

var q =

From O in Db. Orders

where o.shipcity = = "London"

O.freight

Select O;

Statement Description: Use where and order by to sort by freight.

3. Descending sort

var q =

from P in Db. Products

P.unitprice Descending

Select P;

4.ThenBy

Statement Description: Sort the customer by using a composite order by order:

var q =

From C in DB. Customers

C.city, C.contactname

Select C;

Description: Sort by multiple expressions, such as by City first, and when City is sorted by ContactName. This sentence is written in a lambda expression like this:

var q =

. (c = c.city)

. ThenBy (c = c.contactname). ToList ();

There is no ThenBy statement in T-SQL, which is still translated as an order by, so you can also use the following statement to express:

var q =

Db. Customers

. (c = c.contactname)

. (c = c.city). ToList ();

It is important to note that when multiple order-by operations are made, the cascade is in reverse order. For descending, replace with the corresponding descending operator.

var q =

Db. Customers

. OrderByDescending (c = c.city)

. ThenByDescending (c = c.contactname). ToList ();

It is necessary to note that the order by type is not supported, and anonymous classes are not supported. Like what

var q =

Db. Customers

. (c = new

{

C.city,

C.contactname

}). ToList ();

will be thrown out of the exception. The error is that the preceding operation has an anonymous class, and then, when followed by, the category is compared. Like what

var q =

Db. Customers

. Select (c = new

{

C.city,

C.address

})

. (c = c). ToList ();

If you want to use the order order (c + + c), the precondition is that the category of the resulting object must be the basic type of the C # language in the preceding step. For example, here City is a string type.

var q =

Db. Customers

. Select (c = c.city)

. (c = c). ToList ();

5.ThenByDescending

Both of these extension methods are used after orderby/orderbydescending, the first thenby/thenbydescending extension method as the second sort basis, and the second thenby/ ThenByDescending is used as a third-digit sort basis, and so on

var q =

From O in Db. Orders

where O.employeeid = = 1

O.shipcountry, O.freight Descending

Select O;

Statement Description: Order orders for EmployeeID 1 are ordered first by sending to the country and then by freight from high to low.

6. With GroupBy form

var q =

from P in Db. Products

Group p by P.categoryid into G

G.key

Select New {

G.key,

Mostexpensiveproducts =

From P2 in G

where P2. UnitPrice = = G.max (P3 = p3. UnitPrice)

Select P2

};

Statement Description: Use ORDER BY, Max, and group by to derive the highest unit price in each category and sort the products by CategoryID.