what is select * from command used for.And why do we give this command before giving delete command

Not knowing your system in detail, I can only comment on the commands themselves and guess what they are used for:

"select * from radacct where username= 'subscriber2' and AcctstopTime= '0000-00-00 00:00:00';"

Selects all data from the table "radacct" (I guess it is Radius Account) for the user "subscriber2" where the AcctstopTime (Account Stop Time?) is set to zero.
(I assume this is used to control that the correct data is selected for the following delete statement!)

"delete from radacct where username= 'subscriber2' and AcctstopTime= '0000-00-00 00:00:00';"

Deletes the data which was selected above from the table "radacct"

"select * from ConnDetails where username= 'subscriber2';"

Selects all data from the table "ConnDetails" (I guess it is Connection Details) for the user "subscriber2" where the AcctstopTime (Account Stop Time?) is set to zero.
(Again I assume this is used to control that the correct data is selected for the following delete statement!)

"delete from ConnDetails where username= 'subscriber2';"

Deletes the data which was selected above from the table "ConnDetails"

I am not sure i n which environment you have these SQL Statements. If they are in a SQL Script, then the output should be
First the data in radacct for the user
Then a statement like "1 row(s) deleted from radacct" or something similar
Then the data for the user in ConnDetails
Then again a message saying something like "1 row(s) deleted from ConnDetails"

If you came across these SQL Statements in some program code then I guess the SELECT is done to collect data to either show on the screen or somewhere or to log the process in some log file etc.

Hope this helps

Cheers

Peter

We are using a Radius server for providing internet connections to ADSL ( Asymmetric Digital Subscriber Line ) users. We are giving these 4 mysql commands to forcefully disconnect a connected user from the Server. What would happen if we do not give the select commands.. and just give the delete commands. I the select command for viewing certain data from the database.... or is it a prerequisite .. before give delete command.

correction

* Is the select command for viewing certain data from the database.... or is it a prerequisite .. before give delete command. ???

normally using SQL to delete registries in the database, it is not necessary to select first. The delete is sufficient. I don't know about your specific situation, but it sounds strange that it would be required to select first.

Thanks Anpanman ... your reply has cleared my doubts... .. i believe in our case we were told to use select command before delete command to make sure we dont type the delete command wrong ( that would be dangerous ). First we are giving the select command . if there is some typo in select command it wouldnt do no harm and we can correct it. now when we are sure that the select command is giving correct result, we just replace 'select *' with 'delete' .