How can a column be altered based on conditions, with the possibility of creating a new column?

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In SQL, the ALTER TABLE statement can be used to add, modify, or delete columns in a table. To alter a column based on conditions, the following steps can be taken:

1. Use the SELECT statement with a WHERE clause to retrieve the data that meets the conditions.

2. Use the CASE statement to create a new column or modify the existing column value based on the conditions. For example, the following statement creates a new column called "new_column" that sets the value based on a condition:

   SELECT column1, column2,
   CASE
    WHEN column3 = 'value1' THEN 'new_value1'
    WHEN column3 = 'value2' THEN 'new_value2'
    ELSE column3
   END AS new_column
   FROM table_name
   WHERE condition;
   

3. If a new column was created, it can be added to the table using the ALTER TABLE statement with the ADD COLUMN clause:

   ALTER TABLE table_name
   ADD COLUMN new_column datatype;
   

4. If the existing column was modified, it can be updated using the UPDATE statement with the same conditions used in step 1:

   UPDATE table_name
   SET column3 = CASE
                  WHEN column3 = 'value1' THEN 'new_value1'
                  WHEN column3 = 'value2' THEN 'new_value2'
                  ELSE column3
                END
   WHERE condition;