根据同一表中的列更新表中的行

Question

如何根据同一表中列中的值更新表中的行？

表A：

 col1 col2 col3 total col_num
 NULL NULL NULL 100   1
 NULL NULL NULL 200   2
 NULL NULL NULL 300   3

更新后的结果：

表A：

col1  col2  col3 total col_num
100   NULL  NULL 100   1
NULL  200   NULL 200   2
NULL  NULL  300  300   3

Answer 1

除非您以某种方式动态地构造 SQL 语句，否则您不能一般地执行此操作。 但是，对于一组封闭的列，您可以使用一堆case表达式：

UPDATE tableA
SET col1 = CASE col_num WHEN 1 THEN total ELSE col1 END,
    col2 = CASE col_num WHEN 2 THEN total ELSE col2 END,
    col3 = CASE col_num WHEN 3 THEN total ELSE col3 END

Answer 2

您可以使用case表达式：

update tablea
set
    col1 = case when col_num = 1 then total end,
    col2 = case when col_num = 2 then total end,
    col3 = case when col_num = 3 then total end

您可能会包含一些逻辑来仅更新非空列

    update tablea
set
    col1 = case when col1 is null and col_num = 1 then total end,
    col2 = case when col2 is null and col_num = 2 then total end,
    col3 = case when col3 is null and col_num = 3 then total end
where
    (col1 is null and col_num = 1)
    or (col2 is null and col_num = 2)
    or (col3 is null and col_num = 3)