Question about this case statement...Never seen it written this way

Question

I have this case statement written by a developer and i am trying to understand what he was trying to accomplish with writing it in this manner
CASE t_CELL
                WHEN 'XML' THEN
                    'Mods'
                ELSE
                    CASE WHEN t_CELL IS NOT NULL THEN
                        t_Cell
                    ELSE
                        CELL
                    END
                END
            = CELL OR CASE t_CELL
                WHEN 'XML' THEN
                    'Amps'
                ELSE
                    CASE WHEN t_CELL IS NOT NULL THEN
                        t_Cell
                    ELSE
                        CELL
                    END
                END = CELL);What is throwing me off is the = CELL that was included..Kindly note, this is just a sample code

Answer 1

Hi,
If x and cell are strings, then I trust you understand what it means to compare them by saying:
x = cellx can be any kind of expression that returns a string.
For example, do you understand what it means if x is a string literal, like this?
'Amps' = cellDo you understand what it means when x is another string column, like this?
t_cell = cellDo you understand what it means when x is a function that returns a string, like this?
TO_CHAR (SYSDATE, 'Dy') = cellIn the code you posted, the CASE expression is just another way of specifying a string value for x:
CASE ... END = cellA CASE expression is just another kind of expression, that is, another thing that can replace x on the left of the = sign.
It looks like the condition you posted is equivalent to:
(     (     t_cell     = 'XML'
     AND     cell     IN ('Amps', 'Mods')
OR     (     t_cell     != 'XML'
     AND     cell     = t_cell
OR          t_cell     IS NULL
)