Question

Does anyone here know the significant difference of single CASE statement with many WHEN against multiple CASE statements?

eg.

SELECT
  blah
  , CASE WHEN x THEN 'abc'
      WHEN y THEN 'deF'
      WHEN z THEN 'gHi'
    ELSE END

  , CASE WHEN x THEN 'abc' ELSE
      CASE WHEN y THEN 'deF' ELSE
        CASE WHEN z THEN 'gHi' ELSE
        END
      END
    END

.

Thanks in advance for the insights!

Answer 1

Not fully sure of what you want to hear but… Multiple WHEN clauses exist so you don’t need to nest CASE statements.

Edit:

To clarify things: both constructs can do the same things, but nested CASEs makes code more messy and unreadable. A similar example:

SELECT
    CONCAT_WS(' ', 'A', CONCAT_WS(' ', 'B', CONCAT_WS(' ', 'C'))) AS nested,
    CONCAT_WS(' ', 'A', 'B', 'C') AS multiple