Having clause

Having clause

Having clause is similar to the where clause but it is used in the conjuction with group by clause in select statement

Group by clause with having clause used to filter the data or recordset.

Having clause folloed by group by and preced order by clause if used.

Following is the position of having clause

Select

From

Where

Group by

Having

Order by

Eg.

select TitleOfCourtesy from Employees

group by TitleOfCourtesy,BirthDate

having BirthDate >= CONVERT(datetime,'1950-10-12',101)

select TitleOfCourtesy from Employees

group by TitleOfCourtesy,BirthDate

having BirthDate >= '1960-10-12'

SQL Joins

SQL Join

SQL join is used to get the data from two or more table in the database.

A relation between the table is maintained by using the PK and FK reference.

 We can retrieve the data from two or more table using joins which containg some common values in both the table.

SQL join types

Inner join -  Retrieve the data from two or more table when there is matching records in both the table.

select a.FirstName,a.LastName, b.OrderID,b.ShipName,b.CustomerID

from

Employees a

inner join

Orders b

on

a.EmployeeID = b.EmployeeID

order by a.FirstName

Left join -  Retrive all the data from left table with matching records on right table

select a.FirstName,a.LastName, b.OrderID,b.ShipName,b.CustomerID

from

Employees a

left join

Orders b

on

a.EmployeeID = b.EmployeeID

order by a.FirstName

Right join – Retrive all the data from right table with matching records on left table.

select a.FirstName,a.LastName, b.OrderID,b.ShipName,b.CustomerID

from

Employees a

right join

Orders b

on

a.EmployeeID = b.EmployeeID

order by a.FirstName

Full join -  retrive data when there is matching records in one of the table.

select a.FirstName,a.LastName, b.OrderID,b.ShipName,b.CustomerID

from

Employees a

full join

Orders b

on

a.EmployeeID = b.EmployeeID

order by a.FirstName

Self join - is used to join a table to itself as if the table were two tables, temporarily renaming at least one table in the SQL statement.

Cartesian join - returns the Cartesian product of the sets of records from the two or more joined tables.

Cursor

Cursor

A cursor is a temporary work area created in the system memory when a SQL statement is executed. A cursor contains information on a select statement and the rows of data accessed by it.

This temporary work area is used to store the data retrieved from the database, and manipulate this data. A cursor can hold more than one row, but can process only one row at a time. The set of rows the cursor holds is called the active set.

There are two types of cursors in PL/SQL:

Implicit cursors

These are created by default when DML statements like, INSERT, UPDATE, and DELETE statements are executed. They are also created when a SELECT statement that returns just one row is executed. 

Explicit cursors

They must be created when you are executing a SELECT statement that returns more than one row. Even though the cursor stores multiple records, only one record can be processed at a time, which is called as current row. When you fetch a row the current row position moves to next row.

The cursor attributes available are

 %FOUND

 %NOTFOUND

 %ROWCOUNT

 %ISOPEN.

The status of the cursor for each of these attributes are defined in the below table. 

Attributes	Return Value	Example
%FOUND	The return value is TRUE, if the DML statements like INSERT, DELETE and UPDATE affect at least one row and if SELECT ….INTO statement return at least one row.	SQL%FOUND
	The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE do not affect row and if SELECT….INTO statement do not return a row.
%NOTFOUND	The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE at least one row and if SELECT ….INTO statement return at least one row.	SQL%NOTFOUND
	The return value is TRUE, if a DML statement like INSERT, DELETE and UPDATE do not affect even one row and if SELECT ….INTO statement does not return a row.
%ROWCOUNT	Return the number of rows affected by the DML operations INSERT, DELETE, UPDATE, SELECT	SQL%ROWCOUNT

For Example: Consider the PL/SQL Block that uses implicit cursor attributes as shown below:

DECLARE  var_rows number(5);

BEGIN

  UPDATE employee

  SET salary = salary + 1000;

  IF SQL%NOTFOUND THEN

    dbms_output.put_line('None of the salaries where updated');

  ELSIF SQL%FOUND THEN

    var_rows := SQL%ROWCOUNT;

    dbms_output.put_line('Salaries for ' || var_rows || 'employees are updated');

  END IF;

END;

In the above PL/SQL Block, the salaries of all the employees in the ‘employee’ table are updated. If none of the employee’s salary are updated we get a message 'None of the salaries where updated'. Else we get a message like for example, 'Salaries for 1000 employees are updated' if there are 1000 rows in ‘employee’ table. 

Use of caching in Asp.net (GidView Demo)

Post Content

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Xml;

using System.Data;

using System.IO;

using System.Web.Caching;

public partial class _Default : System.Web.UI.Page

{

    string strXmlAllDetails = string.Empty;

    protected void Page_Load(object sender, EventArgs e)

    {

        if (!IsPostBack)

        {

            getData();

        }

    }

    public DataSet getData()

    {

        DataSet dsAllDetailsOutput = (DataSet)Cache["mydata"];

        dsAllDetailsOutput = new DataSet();

        WebServiceDemoExample objWebServiceDemoExample = new WebServiceDemoExample();

        string strXmlOutput = objWebServiceDemoExample.FetchAllDetailsAdmin();

        XmlReader objXmlReader = System.Xml.XmlReader.Create(new StringReader(strXmlOutput));

        dsAllDetailsOutput.ReadXml(objXmlReader);

        Cache.Insert("mydata", dsAllDetailsOutput, new CacheDependency(Server.MapPath("")), DateTime.Now.AddHours(12), Cache.NoSlidingExpiration);

        if (dsAllDetailsOutput.Tables.Count > 0)

        {

            if (dsAllDetailsOutput.Tables[0].Rows.Count > 0)

            {

                GridView1.DataSource = dsAllDetailsOutput;

                GridView1.DataBind();

            }

        }

        return dsAllDetailsOutput;

    }

    protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)

    {

        DataSet ds = (DataSet)Cache["mydata"];

        GridView1.PageIndex = e.NewPageIndex;

        GridView1.DataSource = ds;

        GridView1.DataBind();

    }

}