• Error message : Cannot convert null to int because it is a non-nullable value type(or) INVALID use of NULL.

• The System is recognized as a complete system “Fruit Bazaar System. The main actors for this system as Admin, Users, ShippingAgent.

• User can Log-in, add, remove items to cart, register, pay.
• Admin can have privileged login, they can change, modify catalog, to maintain the user data. In cart adding items then they can make an order. Un order database orders are stored.
• The orders are processed and items from the warehouse are delivered to customer by shipping agent.

• CartCustomer/User can browse the catalog and can add items to their cart for ordering. Moreover, Customer can remove items from their cart and after adding items to their cart they can checkout and move to Ordering.
• For Order a customer should to be log-in.
• Registration Maintenance
• Admin can maintain the Catalog. The Catalog contains all items they are in stock with their rates.
• Dynamic rates according to market value can also change the catalog information by Admin. He can add and remove the items from catalog and stocks.

• User can manage their profile. Profile handling includes changing their account information like email, shipping address, passwords.
• If the user has created an invalid account then Admin can be removing the account or can intimate User about any fault in their profile.
• Physical Transaction At the time of delivery A shipping Agent which will take the items from warehouse along with the bill and deliver the item to customer.
• Customer will pay the respective bill and the bill id will be stored in transaction database.
• Cash transaction should be keeping record like in transaction database which will store bill amount and bill id.
• LOGINUser enter their login-id along with password. If the entered data is valid combination then homepage will be displayed.
• Otherwise a message intimating user about them invalidate login details.
• After placing Order Customer will be intimated by an order confirmation mail and sending a SMS containing order details and delivery time.

• Magic code is used to describe the code which handles the critical task without showing the complexity to present a simple interface.
• Some special characters is called magic code, as single quote( ‘ ), double quote (“), amperson (&) etc. It’s useful for beginner programmer.

• Error handling is the process of catching errors elevated from program and then taking correct action. If you would handle errors then the most common error checking methods given in PHP.
  • die() function
  • Custom errors and error triggers
  • Error reporting

• To write the PHP program you should check all possible error condition before checking and take appropriate action when required.
• The following example without having /tmp/test.xt file and with this file.

• We can write own function to handling any error. PHP provides a framework to define error handling function.
• This function must be able to handle a minimum of two parameters (error level and error message) but can accept up to five parameters (optionally: file, line-number, and the error context)

• These error report levels are the different types of error the user-defined error handler can be used.
• The errors are .E_ERROR, E_WARNING, E_PARSE, E_NOTICE, E_CORE_ERROR, E_CORE_WARNING, E_USER_ERROR, E_USER_WARNING, E_USER_NOTICE, E_STRICT, E_RECOVERABLE_ERROR, E_ALL
• These all errors handle to using set_error_handler function.

• We add a counter in that page.
• When a page load that counter value will increase.
• For storing its previous value we can use session, database etc.

• Indexing is the One of the ways that will optimize the database searching and accessing is having indexes on the columns that access the data in a database table.
• An index object is created in database with the column/columns value that are mentioned while creating the index.
• The value in the index holds the row id of the physical data location in the database which helps for fast data retrieval of the corresponding rows.

• Clustered indexes define the physical sorting of a database table’s rows in the storage media. For this reason, each database table may have only one clustered index.
• Non-clustered indexes are created outside of the database table and contain a sorted list of references to the table itself.

• Indexes their use in queries usually results in much better performance.
• Make it possible to quickly retrieve (fetch) data.
• They can be used for sorting. A post-fetch-sort operation can be eliminated.
• Unique indexes guarantee uniquely identifiable records in the database.

• They decrease performance on inserts, updates, and deletes.
• It will take up space (this increases with the number of fields used and the length of the fields).
• Some databases will monocase values in fields that are indexed.

• Normalization is the process of reorganizing data in a database. They have two basic requirements they are:
  • No redundancy of data.
  • Logical as data dependencies.
• To normalize a database, we divide the database into tables and establish relationships between the tables.
• Database normalization can essentially be defined as the practice of optimizing table structures.
• Optimization is accomplished as a result of a thorough investigation of the various pieces of data that will be stored within the database, in particular concentrating upon how this data is interrelated.

• 1NF
• 2NF
• 3NF
• BCNF
• 4NF
• 5NF

• The First Normal Form should be following some rules in table,
  • It should only have single(atomic) valued attributes/columns.
  • Values stored in a column should be of the same domain
  • All the columns in a table should have unique names.
  • And the order in which data is stored, does not matter.

• For a table to be in the Second Normal Form,
  • It should be in the First Normal form.
  • And, it should not have Partial Dependency.

• A table is said to be in the Third Normal Form when,
  • It is in the Second Normal form.
  • And, it doesn’t have Transitive Dependency.

• Boyce and Codd Normal Form is a higher version of the Third Normal form. This form deals with certain type of anomaly that is not handled by 3NF. A 3NF table which does not have multiple overlapping candidate keys is said to be in BCNF.
• For a table to be in BCNF, following conditions must be satisfied:
  • R must be in 3rd Normal Form and, for each functional dependency ( X → Y ), X should be a super Key.

• A table is said to be in the Fourth Normal Form when,
  • It is in the Boyce-Codd Normal Form.
  • And, it doesn’t have Multi-Valued Dependency.

• Fifth Normal Form (5NF), also known as project-join normal form (PJ/NF) is a level of database normalization designed to reduce redundancy in relational databases recording multi-valued facts by isolating semantically related multiple relationships.
• A relation R is in Fifth Normal Form (5NF) and only if the following conditions are satisfied simultaneously,
  • R is already in 4NF.
  • It cannot be further non-loss decomposed.

• Hierarchical data is a common relational data pattern for representing tree-like data structures, such as an organizational structure, a project breakdown list, or even a family tree.

• In relational database model, One thing to remember is that flexibility in keys.
• We have the flexibility to design the database, it might be worth having multiple geometry types for certain data types.
• For example, having a city represented as a polygon would be useful to perform spatial queries to determine all points of interest within that city.
• At the same time, it is oftentimes more desirable to represent a city as a point on a map. The database allows you to do this by separating the attribute data from the features into their own table, and linking them together with a Primary Key/Foreign Key structure.

• A Primary Key is an identifier for a record in a table.

• A Foreign Key is a reference to that same identifier in a different table, thus linking them together.

• We want to includes this tables to the database:
• Continent – Fields – Geom (Polygon), ID(Primary Key), Name, Etc

Country – Fields – Geom (Polygon), ID, Name, Continent ID(Foreign Key)

• If there are different levels of regions within a country, then we would have them as different layers.
• So, if a country has regions called districts, and a number of districts combine to form a state, which then combine to form the country, you would have a layer for each type.
• Reg_State – Fields – Geom (Polygon), ID (Primary Key), Name, Country ID, Etc

• Have a table that contains attributes for each city, but no geometry.
• Have geometry tables with different geometries, like point or polygon, that link to the main city table.
• City_Info – Fields – CityID (Primary key), Name,Reg_District ID (Foreign Key), Etc,

City_Pt – Fields – Geom (Point), ID, Name, CityID (Foreign Key)

City_Poly – Fields – Geom (Polygon), ID, Name, CityID (Foreign Key)

This is the one of the ways of setting up the database, again it depends on your specific needs for being able to link different pieces of data together.

• Once the table structure is determined, the next part will be to set up queries to retrieve the information you want, for inclusion in a report or some sort of viewing application.

• A database management system (DBMS) is a software package designed to define manipulate, retrieve and manage data in a database.
• A DBMS mostly manipulates the data itself, the data format, field names, record structure and file structure. It also defines rules to validate and manipulate this data.
• A DBMS relieves users of framing programs for data maintenance. Fourth-generation query languages, such as SQL, are used along with the DBMS package to interact with a database.
• Some other DBMS examples include:
  • MySQL
  • SQL Server
  • Oracle
  • dBASE
  • FoxPro

• Data abstraction and independence.
• Data security.
• A locking mechanism for concurrent access.
• An efficient handler to balance the needs of multiple applications using the same data.
• The ability to swiftly recover from crashes and errors, including restartability and recoverability.
• Robust data integrity capabilities.
• Logging and auditing of activity.
• Simple access using a standard application programming interface (API).
• Uniform administration procedures for data.

• One of the most advantages of using a DBMS is that it lets end users and application programmers access and use the same data while managing data integrity.
• Data is better protected and maintained when it can be shared using a DBMS instead of creating new iterations of the same data stored in new files for every new application.

• DBMS is represented by the DataBase Management System. A database management system (DBMS) is a software that allows creation, definition and manipulation of database, allowing users to store, process and analyze data easily.
• DBMS provide us with an interface or a tool, to perform various operations like creating database, storing data in it, updating data, creating tables in the database and a lot more.
• DBMS also provides protection and security to the databases.
• It also maintains data consistency in case of multiple users.
• Here are some examples of popular DBMS used these days:
  • MySQL
  • Oracle
  • SQL Server
  • IBM DB2
  • PostgreSQL
  • Amazon Simple DB (cloud based) etc.

• Hierarchical Databse Model.
• Network Databse Model.
• Relational Databse Model.
• Object-Oriented Databse Model.

• In a Hierarchical database model, the data is organized in a tree-like structure.
• Data is Stored Hierarchically (top down or bottom up) format.
• Data is represented using a parent-child relationship that are one to one or one to many.
• In Hierarchical DBMS parent may have many children, but children have only one parent.

• The network database model allows each child to have multiple parents.
• If you need to address the model help us to more complex relationships like as the orders/parts many-to-many relationship.
• In this model, the entities are organized in a graph which can be accessed through several paths.

• Relational DBMS is the most widely used DBMS model because it is one of the easiest.
• This model is based on normalizing data in the rows and columns of the tables.
• Relational model stored in fixed structures and manipulated using SQL.

• In Object-oriented Model data stored in the form of objects.
• The structure which is called classes which display data within it. It defines a database as a collection of objects which stores both data members values and operations.

• Improved data sharing.
• Improved data security.
• Better data integration.
• Minimized data inconsistency.
• Improved data access.
• Improved decision making.
• Increased end-user productivity.

• Increased costs.
• Management complexity.
• Maintaining currency.
• Frequent upgrade/replacement cycles.

• Lazy write is an effect caused by the use of a write-back cache. In a write-back cache, data is first written to the cache and then is later (during a particular time such as an idle machine cycle) is written to the disk or memory.
• This type of cache is more complex to implement, due to the need to track each locations that have been written over and what needs to be marked as dirty to fix later.

Lazy writer in Memory

• Lazy writer is a thread which is present for each NUMA node (and every instance has at least one) that scans through the buffer cache associated with that node.
• The lazy writer thread sleeps for a specific interval of time, and when it wakes up, it examines the size of the free buffer list.
• If the list is below a certain threshold, which depends on the total size of the buffer pool, the lazy writer thread scans the buffer pool to repopulate the free list.
• As buffers are added to the free list, they are also written to disk if they are dirty.
• So if there is no memory pressure and memory notification is stable it sleeps
• The sole purpose of Lazy Writer is to maintain some free buffers in the SQL Server Buffer Pool.
• Lazy writer runs periodically and check which buffers can be flushed and returned to the free pool.
• So even SQL Server is not under memory pressure, it will also work regularly.

• The inner query is executed only once.
• The inner query will get executed first and the output of the inner query used by the outer query.
• The inner query is not dependent on outer query.

• The outer query will get executed first and for every row of outer query, inner query will get executed.
• So the inner query will get executed as many times as number of rows in result of the outer query.
• The outer query output can use the inner query output for comparison. This means inner query and outer query dependent on each other.

• • The union operator combines the results of two or more queries into a single result set. But no.of columns must match in both/all the queries (and also the order) which are used for union.
  • We can union two tables irrespective of common field.
  • The UNION set operator is used for combining data from two tables which have columns with the same datatype.
    • Union – Returns with no duplicate rows.
    • Union all – Retruns with duplicate rows (No. of rows returned = No. of rows in Query1 + No. of rows in Query 2).

• We cannot use the union operator within a create view statement.
• We cannot use the union operator on text and image columns.

• Joins are used to extract information from more than one table based on the related column/coloums (PK and RFK) any no. of rows can be retrived based on matching colums.
• We can join two tables by ‘join’ if they have common field.
• A Join is used for displaying columns with the same or different names from different tables.
• Different types of joins are Inner join=equi join=join , outer join (Right outer join/right join, Left outer join / left join), Cross join , and full outer join/outer join.
• We can use join operator within a create view statement.
• We can use the join operator on text and image columns.

• SQLcode -501 pops up to show that , the Cursor identified in fetch or close statement is not opened.

• SQL error code means it return codes that are preceded by a minus sign (-) indicate that the SQL statement execution was unsuccessful.
  • If SQLCODE = 0, execution was successful.
  • If SQLCODE > 0, execution was successful with a warning.
  • If SQLCODE < 0, execution was not successful.
  • SQLCODE = 100, “no data” was found.

• We have two tables Wikitechy_Employee and Department. Wikitechy_Employee contains details of employees e.g. id, name, age, salary and department id, while Department contains id and department name.
• In order to show, which employee works for which department we need to join two tables on DEPT_ID to get the the department name.
• Our requirement is to find how many employees are working in each department and average salary of department.
• In order to use WHERE clause, we will only include employees who are earning more than 5000.
• Before executing our query which contains WHERE, HAVING, and GROUP BY clause, let see data from Wikitechy_Employee and Department table:

• From the number of employee (NUM_EMPLOYEE) column you can see that only Vijay who work for Marketing department is not included in result set because his earning 5000.
• This example shows that, condition in WHERE clause is used to filter rows before you aggregate them and then HAVING clause comes in picture for final filtering, which is clear from following query, now Marketing department is excluded because it doesn’t pass condition in HAVING clause i..e AVG_SALARY > 7000