Saturday, September 19, 2020

SQL Homework Help

SQL is a database language. It is designed to help programmers retrieve and manage data in relational databases. SQL is the acronym of structured query language. It is the standard language used by all relational database management systems including MS Access, MySQL, Sybase, Oracle, Postgres, SQL Server, and Informix. However, the language has a variety of dialects. For example, Microsoft SQL Server uses T-SQL, Oracle uses PL/SQL, MS Access uses the JET SQL version, etc.

Stored Procedures   Converting to 3NF Triggers  Designing a Database Schema

Why is SQL popular?

  • SQL is used to create databases and tables
  • It can be used to describe data
  • It allows data definition and manipulation in the database
  • SQL allows the creation and viewing of stored procedures and functions in the database
  • This language can be embedded with other languages by using SQL modules, pre-compilers, and libraries
  • Users can set access restrictions on tables, procedures, and views
  • SQL is an excellent language used to access data in relational databases

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The History of Structured Query Language

Dr. E.F. Codd “Ted” of IBM invented relational databases in 1970. He came up with the first relational model for databases. In 1974, SQL was crafted. Four years later, a product was released named SystemR which was based on Codd’s ideas. The first prototype of a relational database was developed by IBM and standardized by ANSI in 1986. However, the first relational database software was released to the market by Relational Software (Now known as Oracle). Hire our SQL experts if you need detailed writing on the history of SQL.

How SQL works

It is the system that determines the best way to carry out your request when you execute an SQL command for any RDBMS. It is the SQL engine that figures out how to interpret the task. There are a number of components involved in the SQL process. They include the query dispatcher, classic query engine, optimization engine, SQL query engine, etc. All non-SQL queries are handled by the classic query engine. Logical files cannot be handled by the SQL query engine.

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The standard commands in SQL

The standard commands in SQL that are used to interact with relational databases are:

  • DELETE
  • DROP
  • SELECT
  • UPDATE
  • INSERT
  • CREATE

Based on their nature, we can classify these commands into groups as shown below:

  • Data definition language
Command  Description
DROP This command deletes objects in the database like an entire table or a view of a table
CREATE It is the command used to create a new table, a view of a table or any other object in the database
ALTER You can use this command to modify existing objects on the database like a table
  • Data Manipulation Language (DML)
Command  Description
DELETE This command deletes records in a database
UPDATE This command is used to modify records
INSERT It is a command used to create records
  • Data Control Language (DCL)
Command  Description
GRANT This command allows the user to access certain features of the database. IT gives the user certain privileges.
REVOKE This command limits the privileges given to the user. It takes back the access permissions  that the user has
  • Data Query Language
Command  Description
SELECT This command retrieves specific records from one or more tables.

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The Concepts in SQL RDBMS

RDMS (Relational Database Management System) is the basis for SQL. All modern databases like MS SQL Server, Oracle, IBM DB2, Microsoft Access, etc. are based on RDBMS.

  • Tables

Tables are database objects where data are stored in RDBMS. A table consists of columns and rows. It can also be seen as a collection of related data entries. A table is the simplest and most common data storage object. Below is an example of WORKERS table

ID NAME AGE ADDRESS SALARY
1

2

3

4

Abhishek

Kumar

Cate

Henry

32

29

25

28

Delhi

Mumbai

Manchester

New York

2000.00

1500.00

600.00

500.00

  • Fields

A field is a small entity of a table. All RDBMS tables are broken down into smaller entities called fields. Also, we can define a field as a column in a table that maintains specific information about every record in that table. In our WORKERS table above, the fields are ID, SALARY, ADDRESS, AGE, and NAME.

  • Record or row

A row of data is also called a record. It is each individual entry in a table. In our WORKERS table above, we have 4 records. Here is an example:

1  Abhishek 32 Delhi 2000.00
  • Column
ADDRESS
Delhi
Mumbai
Manchester
New York

The vertical entity in a table is known as a column. It has all the information associated with a specific field in a table. An example from our WORKERS table would be:

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The NULL value

In a table, the NULL value is the field that appears to be blank. A field with no value is considered a field with a NULL value. Please note that the NULL value is different from zero value or a field that contains spaces. A NULL value field is one that has been left blank when a record is being created.

Constraints of SQL

Constraints of SQL are the rules that are enforced on the data columns of a table. These regulations limits (restricts) the type of data that can go into a table. SQL constraints guarantee the accuracy and reliability of the data in the database.

We can classify constraints as column level or table level. Table level constraints apply to the entire table, whereas, column-level constraints apply to only one column in the table. According to our SQL project help professionals, the constraints mentioned below are commonly used and available in SQL:

Constraint Description
NOT NULL It makes sure that a column cannot have a NULL value
DEFAULT This constraint provides a default value when none is specified
UNIQUE It is a constraint that ensures that all the values in a column are different
PRIMARY KEY This constraint uniquely identifies each row or records in a table
FOREIGN KEY It is used to uniquely identify rows or records in another database table
CHECK CONSTRAINT This constraint ensures that the values in a column meet the specified conditions
INDEX It is used to quickly retrieve data from the database

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Primary Key

The field in a table that uniquely identifies each row or record in a database table is called a primary key. It cannot have NULL values and must contain unique values. A database table can only have one primary key which has multiple or single fields. When we use multiple fields as a primary key, the fields will be known as a composite key. You can have two records having the same value of fields if your table has a primary key defined on any of the fields. You can send us a message saying “do my SQL homework” if you need help creating a primary key for your table.

How to delete a primary key

We can use the syntax below to clear the primary key constraints from our WORKERS table:

ALTER TABLE WORKERS DROP PRIMARY KEY;

Foreign Key

If you want to link two tables together, then you have to use a foreign key. This concept is called referencing key. A foreign key can be defined as a column or a combination of columns in a different table whose values match the primary key. In SQL, you cannot have two records having the same value as the primary key fields if you have already defined the primary key in the table.

Dropping a foreign key

Suppose that we have another table called ORDERS and we have linked it to our WORKERS table using a foreign key. We can use the following syntax to drop the foreign key:

ALTER TABLE ORDERS DROP FOREIGN KEY;

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