I will explain step by step procedure to create a DB connect connection from SAP BW to external oracle database.
We can have different external database connection to BW like oracle, SQL, DB2, SAP DB etc.
Go to Rsa1 tcode in the BW system as below
Go to source systems in left pane and do right click and click create

Select database system as below

Go to Bd54 tcode
Create logical system similar to highlighted one below. This logical system is a reference and will be used in RSA1 tcode.

Give MPTORA(which ever logical system related to the scenario) in logical system and source system description in RSA1.

and we get below screen
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Fill details like DBMS will be ORA for oracle or MSS for SQL server etc

user id and password we can get it from BW team or from the requester. This user id and password will be used to lo-gin to the external database.
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connection info field will contain the parameter to be used while connecting to the external database.
we need to maintain the details in tnsnames.ora file under /oracle/SID/112*/network/admin. Sample tns file below.
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Save in above screen and go back
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Hit back in above screen

Source system connection created. Right click and do test, the result should be success.

Possible errors:
1. When tnsnames.ora file is not maintained, we will get below error

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Connection info was not maintained so got the above issue.
2. If the user id and password is wrong, we will get below error.
