Which SQL command can be used to add a new column to an existing table without losing data?

The SQL command that can be used to add a new column to an existing table without losing data is the ALTER TABLE command. Here is a step by step guide on how to use it:

1. First, you need to know the name of the table you want to alter. Let's assume the table name is "Students".

2. Decide on the name and data type of the new column you want to add. Let's say we want to add a column named "Email" and the data type is VARCHAR(255).

3. Now, you can construct the SQL command. The general syntax is:

   ALTER TABLE table_name ADD column_name column_type;

4. So, in our case, the SQL command would be:

   ALTER TABLE Students ADD Email VARCHAR(255);

5. Run this command in your SQL environment to add the new column to the "Students" table.

Remember, this command will not remove or alter any existing data in the table. It simply adds a new column. The new column will be empty (NULL) for all existing rows.