更新表与子查询中的表相同

Question

UPDATE MyTable SET Status = (SELECT CASE WHEN COUNT(*) > 0
                            THEN 0          
                            ELSE 1
                            END 
                    FROM MyTable 
                    WHERE Status = 1 )
WHERE RowNumber BETWEEN 1 AND 5
ORDER BY RowNumber

What I want to do is there will be only one row in MyTable with Status = 1. While I update RowNumber 1 to 5, if there has been an existing record with Status = 1, those RowNumber 1 to 5 will be updated with 0. Otherwise, only RowNumber 1 will be updated to 0 and RowNumber 2 to 5 will be set 0.

But aforementioned query is not working. I guess sub-query is run before the whole query and the result from the sub-query is static which means not giving new result as one record after another is being updated.

With this query, if there is no row in the table with Status = 1 then all RowNumber 1 to 5 are updated with Status = 1. This is not what I am expecting.

Answer 1

Not sure if I understand fully what you're after - but it sounds like you need to add a nested case statement in the else clause of your original case. See the code below:

UPDATE MyTable SET Status = (SELECT CASE WHEN COUNT(*) > 0
                        THEN 0          
                        ELSE CASE WHEN RowNumber = 1 THEN 1
                             ELSE 0 END
                        END 
                FROM MyTable 
                WHERE Status = 1 )
WHERE RowNumber BETWEEN 1 AND 5
ORDER BY RowNumber