Question

考虑以下（伪）代码。

int aMin = 10
int bMax = 0

Row selectedRow

foreach(row in rows):
  if (row.a <= aMin && row.b >= bMax)
    aMin = row.a
    bMax = row.b
    selectedRow = row

selectedRow

是否可以编写与此相关的SQL等效项，跟踪当前最低值a和最高值a，以找到与该标准匹配的单行？

Answer 1

DECLARE @aMin int = 10
DECLARE @bMax int = 0 
DECLARE @a int
DECLARE @b int

DECLARE table_cursor CURSOR FOR 
(
SELECT a,b,--maybe more columns here
FROM your_table
)

OPEN table_cursor
FETCH NEXT FROM table_cursor INTO @a, @b

WHILE @@FETCH_STATUS = 0  
BEGIN  

if @a < @aMin and @b > @bMax 
BEGIN
UPDATE your_table -- **PLEASE NOTE HERE THIS UPDATE IS RISKY TO EXECUTE, YOU MAY WANT TO RETURN MORE COLUMNS IN THE CURSOR TO MAKE THE ROWS FOR UPDATING UNIQUE**
SET a = @Min, b = @bMax
WHERE a = @a and b = @b
END

SELECT a,b, --maybe more columns here
FROM your_table 
where a = @a and b = @b

FETCH NEXT FROM table_cursor INTO @a, @b
END

CLOSE table_cursor
DEALLOCATE table_cursor

我认为更好的表现是为每行添加标识列，以便在游标更新中，您可以按Where a = @a and b = @b and identity_row = @i <-- another variable you declared for storing row identifier找到唯一的行

通过检查其他行来查找与动态标准集匹配的单个行

1 个答案: