1. Data Definition Language
Randy Moss

2. Agenda Learn how databases are created
Create and alter the field structure of tables

3. Database Definition Language (DDL)
Subset of SQL commands to create and modify database objects
Can create an entire database using SQL rather than a graphical user interface
With DDL you gain more control over exactly what database obejcts are built and what they are named
DDL saved into a script can be easily ported from one database installation to another

4. Naming tips Using consistent names
Do not include spaces in your table or fields
Avoid the use of SQL reserved words and punctuation marks in table and field names
Keep names short, but long enough to be descriptive
Be consistent in your use of abbreviations
Name primary key and foreign key fields consistently
Avoid generic field names, such as ID or Date

5. Creating a Table Syntax: CREATE TABLE tableName ( )
fieldname datatype null | not null,
fieldname datatype null | not null )

6. Example of Creating a Table
CREATE TABLE Contracts ( ArtistID Integer NOT NULL, ContractDate DateTime NOT NULL );

7. Dropping a Table
DROP TABLE tableName; Example: DROP TABLE Contracts;

8. Adding Columns to an Existing Table
Syntax: ALTER TABLE tableName
ADD columnName dataType null | not null;
Example:
ALTER TABLE Contracts
ADD ContractType VarChar(10) NULL;

9. Altering Existing Columns in a Table
Syntax: ALTER TABLE {tableName}
MODIFY COLUMN {columnName} {dataType} {null | not null};
Example: # Change the length on the ContractType field to 15 in the Contracts Table
ALTER TABLE Contracts
MODIFY ContractType VarChar(15);

10. Dropping a Column Syntax: ALTER TABLE {tableName}
DROP COLUMN {columnName};
Example:
# Remove the ContractType column from Contracts table
ALTER TABLE Contracts
DROP COLUMN ContractType;

11. Columns and Sequences
Techniques to automatically generated a sequential number with each row that is inserted
Never have to insert a value to it, but it will always be unique within the table
Make excellent primary key in cases where there is no naturally occurring primary key

12. Column Sequence Example
CREATE TABLE tbl ( ID int AUTO_INCREMENT NOT NULL, anotherField int null, primary key(ID) );

13. Sequences Vs. Guid Columns
An identity column (or sequence) is unique within a given a table in a given database on a given server
With distributed database, identity column may not\be unique across every instance of the table
One solution is to use the identity column in conjunction with a locationID as the primary key
Another solution Globally Unique Identifier (GUID), which is supported in some databases
A GUID is a 16-byte ID guaranteed to be unique across every instance of a distributed database
GUIDs are cumbersome to work with: C1C5FCE2-879E-4A96-B845- 9F3FC4A10EC8