Question

Consider following table

--------------------------------
ID   | ColA    | ColB   | ColC
--------------------------------
1    | ABC     |        |
2    |         | XYZ    |
3    | PQR     |        |
4    | MNO     | PQR    |

Now I have to add a value (EFG) in this table where ID = 1 in any column like ColA, ColB or ColC depending on which is free (empty). This is done dynamically or programmatically (consider any language, I just want to know the logic).

For example:

If ID = 1, column ColB will be updated with value because it’s the first free column.
If ID = 2, column ColA will be updated with value because it’s the first free column.
If ID = 3, column ColB will be updated with value because it’s the first free column.
If ID = 4, column ColC will be updated with value because it’s the first free column.

Answer 1

since the record is already inserted, you only need to update the row,

UPDATE  tableName
SET     ColA = CASE WHEN ColA IS NULL THEN 'NewValue' ELSE ColA END,
        ColB = CASE WHEN ColA NOT NULL AND ColB IS NULL THEN 'NewValue' ELSE ColB END,
        ColC = CASE WHEN ColA NOT NULL AND ColB NOT NULL AND ColC IS NULL THEN 'NewValue' ELSE ColC END
WHERE   ID = 1