The enterprise integration is to exchange data cross different database and platform. Most of the integrations deal with asynchronous messaging architectures. Enterprise Integration Patterns help you design better solutions whether you use EAI, SOA, or ESB platforms such as IBM WebSphere MQ, TIBCO, Vitria, SeeBeyond (now Sun), WebMethods, BizTalk, Sonic, Fiorano, Mule ESB etc., JMS-based messaging systems, MSMQ, WCF, or Web service-based integration.

Here I am discussing database integration.

source database: ERP database or external database .
target database: enterprise database.
integration client: install at local machine to receive, transfer and communicate with server.
integration server: take request from client to extract , transform and load data into target database.

1. Database Trigger and Alert Notification
Create a source database trigger to catch event and use system tool (i.e. xp_cmdshell) to notice client. When client gets notification, it will send message (i.e. JMS or SOAP) to server. (see my example here)

2. Database Trigger and Shadow table
Create copy of table you specified. When you are working with Trigger type events, once you have configured a channel rule, a trigger keeps copying the event type (Insert/Delete/Update) rows into another Table from the Table you specified when you defined the channel rule. See example of BEA WebLogic Integration-RDBMS events.

3. Hosted Web Services as middleware
Build a bridge between source and target database through server (middle layer). Client can support UI platform to map to associated field. See example of JitterBit.

4. Pull/Batch to extract data
Create a task scheduler to pull data from source database to target database via server. Client can set up timer to extra data.

5. Database SOAP/HTTP
Create a stored procedure to generate HTTP POST to send message to server. All data extraction will implement at database side. See example of Overview of Native XML Web Services for Microsoft SQL Server 2005.