QlikView Interview Question and Answers-3

Question.1  What is the difference between Star schema and Snow flake scheme ?

Answer:

In Star schema, Fact table is in the center and surrounded by dimension tables. Snow flake schema is similar to star schema. In Snow flake schema, dimension tables may be connected to other related dimensions. For example, City dimension may connect to address dimension.

Question.2  How data is stored internally in QlikView ?

Answer:

QlikView internally stores data in two levels. First level stores distinct lists of value and the second level contains pointers to these values.

Data is stored in a field only once. For example, if the City is present in multiple tables in your database, it will be stored only once in QlikView. This is very useful in large datasets, where repeated values are stored only once.

Question.3  How data is extracted in QlikView ?

Answer:

In QlikView, data is extracted by writing scripts. Various load statements are used in scripts to extract data.

Scripts are written in Script editor. To write scripts, launch QlikView desktop and press Ctrl + E to invoke the script editor.

Data can be extracted from the database by using an OLEDB/ODBC connection or from files such as excel, text files or QVDs.

Question.4  What steps should be followed to load the database tables in QlikView ?

Answer:

To load data from a database table,

  • Launch QlikView desktop. Use Ctrl + E to initiate script editor.
  • Create an ODBC or OLEDB connection to the database.
  • In the script editor, from the bottom tabs, select Data tab and click on connect.
  • Create database connection and select tables.
  • Once the table is selected Load script is automatically generated.
  • As a best practice, always name your table. The Table is referenced by Table name throughout the script. The syntax is TableName: or [Table Name]: depending on if the table name contains a space or no space.
  • Click on the Reload icon from the menu or Ctrl + R to load data.
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Question.5  What steps should be followed to load table files such as excel, text file or QVDs ?

Answer:

To load the table files,

  • Launch QlikView desktop. Use Ctrl + E to initiate script editor.
  • In the script editor, from the tab at the bottom select Table files and browse to your files.
  • Once the file is selected, load statement is automatically generated.
  • As a best practice, always name your table. The Table is referenced by Table name throughout the script. The syntax is TableName: or [Table Name]: depending on if the table name contains a space or no space.
  • Click on the Reload icon from the menu or Ctrl + R to load data.

If TestTable.xlsx, mentioned in the beginning of this book is loaded, the load script will
look like

TestTable: //Name of the table
LOAD

ID,
Year,
Name,
Address,
Product,
Amount

FROM
TestTable.xlsx
(ooxml, embedded labels, table is TestTable);

TestTable in the last line, is the name of the sheet tab in excel

Question.6  How loaded tables and data model can be viewed ?

Answer: 

Once the data is loaded, use Table viewer to see the loaded table.

  • Table viewer can be invoked by clicking on the Table Viewer icon from the menu or by using Ctrl + T.
  • Table viewer displays the loaded table with fields. If multiple tables are loaded, it shows how tables are linked.
  • You can get a preview of the table data. Hover over the table, to display other important attributes of the table.

Question.7  What approach should be followed in creating a data model ?

Answer:

  • The data model is created by extracting data and applying transformations.
  • The data model should be designed by using QVDs (QlikView Data Files). A table of data should be extracted and stored in a QVD. While creating visualization, data should be read from the QVD as reading/writing data from QVD provides better performance.
  • A multi-tier architecture should be followed while creating data model. This involves creating a data layer to extract raw data from tables and storing it in QVD.
  • In Transform layer, data transformation is applied.
  • In Presentation layer, data from the QVDs are read to create visualizations.
  • While loading data from multiple tables, synthetic keys and loops may be formed. Synthetic keys and loops should be resolved.
  • Data model should be clean with less number of tables.

Question.8  What problems can occur when multiple tables are extracted in QlikView ?

Answer:

When multiple tables are loaded in QlikView, sometimes Synthetic keys or loops are formed.

Question.9  What is synthetic key and how it is resolved ?

Answer:

Synthetic key is formed by the presence of more than one common field between the two more tables.

There are different ways to resolve a synthetic key. Depending on the specific scenario, any of the following techniques can be used

  • Ensure that only required fields connect.
  • Alias the column that is not required to be linked.
  • Comment column/s in a table that are not required in the dashboard.
  • Use Qualify and UnQualify statements.
  • Concatenate tables.
  • Create Link table.
  • Concatenate key fields to create a composite key.

Question.10  What is a QVD ?

Answer:

  • QVD stands for QlikView Data File. It contains the data extracted from the data source table.
  • QVD is a native QlikView format. Read/write to QVD can only be done from QlikView.
  • Reading data from a QVD file is faster than reading from a database table.
  • QVD can store any kind of data viz. Database, excel or text file. QVD file has .qvd extension. It is created using the Store command.
  • Data is read from QVD using the load command

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