SQL

SQL

1. A Query is a search on the database.

   • SQL stands for structured query language. A query is written in SQL and sent to the database query engine as plain text.
   • The database executes the query and returns the results in the form table.
   • Common Constructs
     • Select, Specifies the data/attributes that need to be accessed.
       • Example: SELECT Surname, FROM STUDENT, WHERE StudentID = 6. Result = Surname is Jones
     • From, Specifies the Table/Tables to search from.
     • Where, Specifies the criteria/condition to be satisfied.
     • Order By, Specifies the Sort Order
     • Group By, Specifies how to group the records.

2. Normalisation

   • The process of splitting a single file into multiple related tables with common keys.
   • 1st Normal Form = Attributes should only have single values. 2nd Normal Form = No partial dependencies of the primary key. 3rd Normal Form = No transitive dependencies.

3. SQL Basics

   • UPDATE
     • It is used to edit or modify existing data in the database.
   • INSERT
     • As well as retrieving data, SQL also lets you add records using the INSERT Command.
   • DELETE
     • Enables you to delete data from a database.
   • JOIN
     • Used to combine records from two or more tables in a database. It is a means of combining fields from two tables by using values common to each.