1)请运行此脚本并阅读我的评论。
2)我希望这个答案会对你有所帮助。
3) SUM()-SUM() 的精度为 2,因为您选择先求和 (SUM(total)
和SUM(rem)
),然后再减去 ( SUM(total) - SUM(rem)
)。
4)我的建议是使用SELECT t.code, SUM(t.total - t.rem) AS diff ...
(先减去然后求和)。
5) 你可以阅读我对这个问题的回答SQL Numeric data type truncating value? :
DECLARE @Test TABLE(
code varchar(15),
total numeric(13,2),
rem numeric(13,4)
);
INSERT @Test (code, total, rem)
VALUES ('001', 11.78, 5.6789);
--Test [1]
SELECT dt.*,
SQL_VARIANT_PROPERTY(dt.diff, 'BaseType') AS diff_BaseType,
SQL_VARIANT_PROPERTY(dt.diff, 'Precision') AS diff_Precision,
SQL_VARIANT_PROPERTY(dt.diff, 'Scale') AS diff_Scale
FROM
(
SELECT t.code, t.total - t.rem AS diff
FROM @Test t
) dt;
/*
Operation: e1 - e2
Result precision: max(s1, s2) + max(p1-s1, p2-s2) + 1 = max(2,4) + max(13-2, 13-4) + 1 = 4 + 11 + 1 = 16
Result scale: max(s1, s2) = max(2, 4) = 4
*/
--Test [2]
SELECT dt.*,
SQL_VARIANT_PROPERTY(dt.diff, 'BaseType') AS diff_BaseType,
SQL_VARIANT_PROPERTY(dt.diff, 'Precision') AS diff_Precision,
SQL_VARIANT_PROPERTY(dt.diff, 'Scale') AS diff_Scale
FROM
(
SELECT t.code, SUM(t.total - t.rem) AS diff
FROM @Test t
GROUP BY t.code
) dt;
/*
Operation: SUM(e1 - e2)
Result precision: 38--For SUM function, I think (it's just a hipotese), SQL Server choose the maximum precision to prevent the overflow error
Argument:
DECLARE @t TABLE (Col NUMERIC(2,1)); INSERT @t VALUES (1);
SELECT SQL_VARIANT_PROPERTY(SUM(t.Col), 'Precision') FROM @t t;
Result: precision = 38 (maximum DECIMAL/NUMERIC precision)
Result scale: the same scale as (e1-e2)= 4 (please see Test [1])
*/
--Test [3]
SELECT dt.*,
SQL_VARIANT_PROPERTY(dt.SUM_total, 'BaseType') AS SUM_total_BaseType,
SQL_VARIANT_PROPERTY(dt.SUM_total, 'Precision') AS SUM_total_Precision,
SQL_VARIANT_PROPERTY(dt.SUM_total, 'Scale') AS SUM_total_Scale,
SQL_VARIANT_PROPERTY(dt.SUM_rem, 'BaseType') AS SUM_rem_BaseType,
SQL_VARIANT_PROPERTY(dt.SUM_rem, 'Precision') AS SUM_rem_Precision,
SQL_VARIANT_PROPERTY(dt.SUM_rem, 'Scale') AS SUM_rem_Scale,
SQL_VARIANT_PROPERTY(dt.diff, 'BaseType') AS diff_BaseType,
SQL_VARIANT_PROPERTY(dt.diff, 'Precision') AS diff_Precision,
SQL_VARIANT_PROPERTY(dt.diff, 'Scale') AS diff_Scale
FROM
(
SELECT t.code,
SUM(t.total) AS SUM_total, SUM(t.rem) AS SUM_rem, SUM(t.total) - SUM(t.rem) AS diff
FROM @Test t
GROUP BY t.code
) dt;
/*
Operation: SUM(total) (<> e1 + e2 + ...)
Result precision: 38--I think SQL Server choose the maximum precision to prevent the overflow error
Result scale: the same precision as total= 2
*/
/*
Operation: SUM(rem) (<> e1 + e2 + ...)
Result precision: 38--I think SQL Server choose the maximum precision to prevent the overflow error
Result scale: the same precision as rem= 4
*/
/*
Operation: SUM(total) - SUM(rem) = e1 - e2
Result precision: max(s1, s2) + max(p1-s1, p2-s2) + 1 = max(2,4) + max(38-2, 38-4) + 1 = 4 + 36 + 1 = 41
but max. precision is 38 so result precision = 38
Calculated result scale: max(s1, s2) = 4
but because the real precision for result (41) is greater than maximum precision (38)
SQL Server choose to decrease the precision of the result to 2 (please see Test [3] - diff_Scale).
In this case (the real precision for result is greater than maximum precision) I think the
expression for result's precision is max(s1, s2) - (real precision - maximum precision) + 1 = 4 - (41 - 38) + 1 = 4 - 3 + 1 = 2
For example you could try to modify the definition of total column to `total numeric(13,1)`
and you will see that the precision for SUM(total) - SUM(rem) becomes 4 - 4(4+37+1=42) + 1 = 1
*/
结果:
--Test [1] SELECT t.code, t.total - t.rem AS diff
code diff diff_BaseType diff_Precision diff_Scale
---- ------ -------------- -------------- ----------
001 6.1011 numeric 16 4
--Test [2] SELECT t.code, SUM(t.total - t.rem) AS diff
code diff diff_BaseType diff_Precision diff_Scale
---- ------ ------------- -------------- ----------
001 6.1011 numeric 38 4
--Test [3] SELECT t.code, ..., SUM(t.total) - SUM(t.rem) AS diff
code SUM_total SUM_rem diff SUM_total_BaseType SUM_total_Precision SUM_total_Scale SUM_rem_BaseType SUM_rem_Precision SUM_rem_Scale diff_BaseType diff_Precision diff_Scale
---- --------- ------- ---- ------------------ ------------------- --------------- ---------------- ------------------------------- ------------- -------------- ----------
001 11.78 5.6789 6.10 numeric 38 2 numeric 38 4 numeric 38 2