Update with case statement problem.

Question

Hi,
I have this statement
update tab_tr set col_rate =
  case
  when col_ern_type in ('031','035','036') then (1.5 * col_rate)
  when col_ern_type in ('041','045','046') then (2 * col_rate)
end;this statement updated some 2 million rows in the table tab_tr. But the total count of rows for col_ern_type in ('031','035','036', '041','045','046') yeild only around 222642 rows.
select count(*) from tab_tr where col_ern_type in ('031','035','036', '041','045','046');  --> 222642 rowswhat is the problem with the above update statement.
Thank you,

Answer 1

Hi,
DBA_1976 wrote:
... I thought the case statement conditions are counted as the where condition. But ......A CASE expression just takes the place of some other expression, such as a literal, a function, or a column.
For example, which rows do you think this should update?
UPDATE  tab_tr
SET     col_rate  = 100;It would update all rows, of course.
How about this statement?
UPDATE  tab_tr
SET     col_rate  = column_x;Will it only change the rows where column_x are a certain value, or not NULL, or in any way depend on column_x? No, of course not. It will update all rows.
How about this statement?
UPDATE  tab_tr
SET     col_rate  = function_y (col_ern_type);Will it only change the rows where the fucntion returns a certain value? Will it depend on the value in col_ern_type? No, of course not. It calls the function for each row, and whatever the function returns (even if the function returns NULL), that's what goes into col_rate on each row.
A CASE expression is just something that you can substitute in place of any other expression, such as the literal 100, or the column column_x, or the function function_y in the statements above. Naturally, the value that gets put into col_rate will depend on what the CASE expression returns, but the behavior of the UPDATE statement as a whole will not.