One day you might be forced to do modifications on live database and accidentially delete/update all records in table instead of specified in where statement. It happened to me in a past it might happen to you too. Usually you think that world is over. Then you remember about backup, but still there is a possibility that some data would be lost forever. Always make sure to email your records to yourself with a trusted email service provider for further back up. The best solution is as usual prevention of disaster.

There is a reason why SSMS Tools pack by default creates for new queries:

BEGIN TRAN

...

ROLLBACK

Use transaction for all your modifications. Inside transaction you can test if your update/delete does exactly what you want it to do.
Here is a little illustration:

CREATE TABLE TestTable

(

    Id int identity primary key ,

    [Description] varchar(max)

)

GO

BEGIN TRAN

insert into TestTable

values

('Test1'),

('Test2'),

('Test3')

select * from TestTable -- First query, returns all records

delete TestTable

where Id=2

select * from TestTable -- Second query, returns two records

ROLLBACK -- COMMIT

select * from TestTable -- Third query, no records because of ROLLBACK

Results:

Query 1:

Query 2:

Query 3:

After you are done with testing, just change your ROLLBACK with COMMIT statement.