Introduction to SQL
A language used in programming and designed for managing data held in a relational database management system.
What is SQL?
Structured Query Language is a standard Database language which is used to create, maintain and retrieve the relational database.
SQL became a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987
What Can SQL do?
SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert records in a database
SQL can update records in a database
SQL can delete records from a database
SQL can create new databases
SQL can create new tables in a database
SQL can create stored procedures in a database
SQL can create views in a database
SQL can set permissions on tables, procedures, and views
SQL is a Standard - BUT....
Although SQL is an ANSI/ISO standard, there are different versions of the SQL language.
However, to be compliant with the ANSI standard, they all support at least the major commands (such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar manner.
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
RDBMS
RDBMS stands for Relational Database Management System.
RDBMS is the basis for SQL, and for all modern database systems such as MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access.
The data in RDBMS is stored in database objects called tables. A table is a collection of related data entries and it consists of columns and rows.
Below table shows the relational database with only one relation called STUDENT which stores ROLL_NO, NAME, ADDRESS, PHONE and AGE of students.
STUDENT
ROLL_NO | NAME | ADDRESS | PHONE | AGE |
1 | RAM | DELHI | 9455123451 | 18 |
2 | RAMESH | GURGAON | 9652431543 | 18 |
3 | SUJIT | ROHTAK | 9156253131 | 20 |
4 | SURESH | DELHI | 9156768971 | 18 |
These are some important terminologies that are used in terms of relation.
Attribute: Attributes are the properties that define a relation. e.g.; ROLL_NO, NAME etc.
Tuple: Each row in the relation is known as tuple. The above relation contains 4 tuples, one of which is shown as:
1 | RAM | DELHI | 9455123451 | 18 |
Degree: The number of attributes in the relation is known as degree of the relation. The STUDENT relation defined above has degree 5.
Cardinality: The number of tuples in a relation is known as cardinality. The STUDENT relation defined above has cardinality 4.
Column: Column represents the set of values for a particular attribute. The column ROLL_NO is extracted from relation STUDENT.
ROLL_NO |
1 |
2 |
3 |
4 |
Types Of SQL Commands
The queries to deal with relational database can be categorized as:
Data Definition Language (DDL): It is used to define the structure of the database. e.g; CREATE TABLE, ADD COLUMN, DROP COLUMN and so on.
Data Manipulation Language (DML): It is used to manipulate data in the relations. e.g.; INSERT, DELETE, UPDATE and so on.
Data Query Language (DQL): It is used to extract the data from the relations. e.g.; SELECT
Data Control Language (DCL) is used to control access to data stored in a database. e.g.; GRANT, REVOKE
Transaction Control Language(TCL) commands are used to manage transactions in the database. e.g.; COMMIT, ROLLBACK, SAVEPOINT.
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