What is DDL (Data Definition Language) in SQL?
DDL commands
are subset of SQL commands. These commands are primarily used for defining and
modifying the structure of a database or a table. These are the following
commands used in DDL:
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| Data Definition Language in SQL |
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Other Topics of this Chapter
- Database
- Primary key / Foreign Key
- Create Database in Open Office Base
- Create Table in Open Office Base
- Update Table in Open Office Base
- Create Query in Open Office Base
- Creating Form in Open Office Base
- Report in Open Office Base
- DDL (Data Definition Language)
- DML (Data Manipulation Language)
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How to use Create command in SQL?
Syntax: Create Table <table name>
(<column name>
<data type> <size>,
<column name> <data type>
<size>, - - - );
Example: Create a table lab which has the following columns:
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| Table - Lab |
SQL> Create
table Lab
( Item_Code Char(3),
Item_Name Varchar(10),
Price Float(4,2),
Quantity Int,
Date_of_Purchase Date);
How to use Alter command in SQL?
Sometimes it may happen that after the table has been created and the values have been entered in a table. we may require to change the definition of the table, so Alter command is used. It is a DDL statement.
Table can be altered in one of the three ways:
1) By adding a column to an existing table,
2) By changing a column’s definition (data type),
3) By dropping a column of the table.
1) Adding column in an existing table
Syntax: Alter table <table name>
Add <column name> <data type> [constraint definition];
Example: SQL> Alter Table Lab
Add Item_Company Varchar(10);
2) Changing a column’s definition
Syntax: Alter Table <table name>
Modify <column name> <data type>;
Example: SQL> Alter Table Lab
Modify Item_Name varchar(15);
3) Dropping a column from the table
Syntax: Alter Table <table name>
Drop Column <column name>;
Example: SQL> Alter Table Lab
Drop Column Item_Company;
How to use Rename command in SQL?
Rename command is used in three ways in SQL:
1) To change Column name in a Table
Syntax: Alter Table <table name>
Rename column <Column old_Name> to <New_Name>;
Example: SQL> Alter Table Lab
Rename column Date_of_Purchase to DOP;
2) To change table name in a Database
Syntax: Alter
Table <table name>
Rename to New_Table_Name>;
Example: SQL> Alter Table Lab
Rename to Shop;3) To change Database name
Syntax: Alter Database
<Existing_Database_Name> modify Name = <New_Database_Name>;
Example:
SQL> Alter Database
How to use Drop command in SQL
Dropping a table not only deletes the data contained in the table but it also removes the definition of its structure. It is a DDL statement.
Syntax: Drop Table <table name>;
Example:
SQL> Drop table Employee;
What is Constraints in SQL?
Constraints are used to prevent invalid data entry into the table. They provide methods so to follow the rules of an organization.
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| Constraints in SQL |
Chapter 1 – Digital Documentation
Chapter 2– Electronic Spreadsheet
Chapter 3 – Database Management System
Chapter 4 –Web Application
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1) Not Null is neither a zero nor a blank space because zero is a number value and blank space is a character. Not Null constraint on a column ensures that the column doesn’t remain empty.
Example:
SQL> Create Table Student
( Name Varchar
(25) Not Null,
Class char(3) );
2) Unique constraint makes it sure that the data entered in a column does not match with the records.
Example: SQL> Create Table Student
( Roll-No char(3) Unique,
Name Varchar(15) Not Null,
Class char(3));
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3) Primary Key constraint makes it sure that data is unique and cell doesn’t remain empty.
Example: SQL> Create Table Student
( Roll-No char(3) Primary Key,
Name Varchar(15),
Class char(3));
4) Check constraint makes sure that data entered in a table is within the certain range of our requirement.
Example: SQL> Create Table Employee
( EmpNo char(3),
Ename Varchar(15),
Salary Float(8,2) Check (salary > 0) );
5) Foreign Key constraint setup a relationship with Primary Key of another table.
Example: SQL> Create Table Student
( Roll-No char(3),
Name Varchar(15),
Class char(2),
Stream Char(5) References
School(Stream));
Chapter 1 – Digital Documentation
Chapter 2– Electronic Spreadsheet
Chapter 3 – Database Management System
Chapter 4 –Web Application
What are the 5 arithmetic operators in SQL?
SQL
statements are made of reserved word or symbols which are used to
perform arithmetic operations. These reserved words are called operators
in SQL.
Operators Description Example
+ Perform Addition 7 + 2 = 9
- Perform Subtraction 7 – 2 = 5
* Perform Multiplication 7 * 2 = 14
/ Perform Division 7 / 2 = 3
% Returns remainder 7 % 2 = 1
What are the comparison operators in SQL?
Comparison operators are used to compare two values and return true (1) or false (0).Operators Description Example
= Less than 7 = 2 False
<= Less than equal to 7 <= 2 False
>= Greater than equl to 7 >= 2 True
<> Not equal to 7 <> 2 True
What are the 5 logical operators in SQL?
Operators Description
AND All conditions should be true.
OR Any condition true.
NOT Reverse the meaning of operator.
BETWEEN Search the value within the set of values.
ALL Compare a value to all values.
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