BTM 382 Database Management

Chapter 7Introduction to SQL(Structured Query Language)

Chitu OkoliAssociate Professor in Business Technology Management

John Molson School of Business, Concordia University, Montréal

Data Definition Language (DDL) andData Manipulation Language (DML)

Data Definition Language (DDL) defines the structure of the database E.g. tables, columns, keys, indexes, etc.

Data Manipulation Language (DML) manipulates the actual data contents E.g. adding, modifying and deleting data

Understanding the difference between these two types of SQL commands will help you master SQL

Four major kinds of database operations:Create, Read, Edit/Update, Delete (CRED/CRUD)Data Definition

Language (DDL):Change database structure

Data Manipulation Language (DML):Change data contents inside tables

Create

CREATE tables and other structures (also ADD)

INSERT data into tables

Read SELECT (system catalogue queries)

SELECT data from tables

Edit/ Update

ALTER table and other structures (also MODIFY)

UPDATE table

Delete DROP tables and other structures

DELETE from table

COMMIT and ROLLBACK in Oracle(DML commands) COMMIT

Saves all DML changes since the last COMMIT or DDL command Whenever you issue any DDL command, COMMIT is automatically

triggered In other words, whenever you issue a command that changes

the structure of the database, all data contents are automatically saved right away

If you do not COMMIT before closing the connection, all uncommitted DML commands (changes to data contents) will be lost

ROLLBACK Cancel all DML changes since the last COMMIT or DDL

command Microsoft Access automatically commits every command (you

cannot undo any changes)

Oracle character types: CHAR and VARCHAR2 CHAR(n)

Always a string of exact length of n (any unused characters are filled with blanks)

E.g. Hello in CHAR(10) is stored as “Hello ” (10 characters) VARCHAR2(n)

A string of maximum length of n (any unused characters are empty) E.g. Hello in VARCHAR2(10) is stored as “Hello” (5 characters)

Converts empty strings (‘’) to null VARCHAR(n)

Same as VARCHAR2, except that it distinguishes between null and empty string (‘’)

Not used; automatically converted to VARCHAR2 for now Might be used in the future, so just don’t ever use it

Best practice (recommendation, not requirement): Only use VARCHAR2 all the time; there is no good reason to ever use anything else

WHERE versus HAVING clauses

WHERE is used to restrict a subset of rows from a regular query result Usually a SELECT query, but also UPDATE and DELETE

HAVING is used to restrict a subset of rows when using a GROUP BY aggregation HAVING only works with GROUP BY

Mnemonics to help you not confuse the two: SELECT FROM WHERE? (WHERE is the normal clause for

a SELECT statement)SELECT column-listFROM tablesWHERE conditions

G-H (GROUP BY goes with HAVING)

Set date and number formats in Oracle

Some data (e.g. dates and numbers) assume that certain formats are being used; using different formats can give errors

ALTER SESSION is used to set environment variables, including date and number formats Changes will last until you end the session (e.g. close Oracle SQL

Developer); then they will revert to the default Date formats: If you have problems entering dates, execute this SQL

command before all your other commands: alter session set nls_date_format = 'dd-mm-yyyy';

You need to customize the date format to exactly what you want

You can also use the function: to_date( '20150301', 'YYYYMMDD' ) Language and number formats: These commands might help:

alter session set nls_language = English; alter session set nls_territory = Canada; alter session set nls_date_format = 'dd-mm-yyyy';