数字乘以常数结果为负数

Question

我正在尝试计算一年前相应一周的日期。它现在似乎可以正常工作，但有一件事让我很困扰，它最初导致了我通过尝试各种方法修复的几个错误。

问题是，当我不使用 to_number 和 &weekOfYearNow - &weekOfYearThen 并使用此结果计算 lastYearFixed 时，结果将是 -311 而不是7。为什么会这样？这两个变量 (weekOfYearNow & weekOfYearThen) 不是已经是数字了吗？

define today = to_date('30/12/20', 'dd/mm/yy');
define lastYear = add_months(&today, -12);

select &today as "Today" from sys.dual;
select &lastYear as "LastYear" from sys.dual;

define weekOfYearNow = to_number(to_char(&today, 'ww'));
define weekOfYearThen = to_number(to_char(&lastYear, 'ww'));
define weekOfYearOffset = to_number(&weekOfYearNow - &weekOfYearThen); -- <-- must use to_number - but why?

define lastYearFixed = &lastYear + (&weekOfYearOffset * 7); -- <-- this equals -311 if I don't call to_number above

define monday = add_months(sysdate - 7 - to_char(sysdate, 'd') + 2, -12);
define sunday = add_months(sysdate - 7 + to_char(sysdate, 'd'), -12);

select &weekOfYearNow as "WeekOfYearNow" from sys.dual;
select &weekOfYearThen as "WeekOfYearThen" from sys.dual;
select &weekOfYearOffset as "WeekOfYearOffset" from sys.dual;
select &lastYearFixed as "LastYearFixed" from sys.dual;

Answer 1

您不需要（也不应该使用）to_number()；但您确实需要将第一个表达式括在括号中，如：

define weekOfYearOffset = (&weekOfYearNow - &weekOfYearThen);
define lastYearFixed = &lastYear + (&weekOfYearOffset * 7);

或

define weekOfYearOffset = &weekOfYearNow - &weekOfYearThen;
define lastYearFixed = &lastYear + ((&weekOfYearOffset) * 7);

如果没有其中任何一个，当替换被扩展时，你有效地得到：

define lastYearFixed = &lastYear + (&weekOfYearNow - &weekOfYearThen * 7);

（它实际上扩展了所有这些术语以显示嵌套的 to_date() 和 to_number() 等调用，但为了简洁起见，这给出了想法；重点是它们还不是数字，并且尚未完成任何转换或计算。您可以 set verify on 查看完整的 glory/horror。）

operator precedence 的 * 优先级高于（二进制）-，这意味着它等同于：

define lastYearFixed = &lastYear + (&weekOfYearNow - (&weekOfYearThen * 7));

所以你要加 weekOfYearNow 并减去 7 次 weekOfYearThen；而不是将它们之间的差值相加 7 倍。

数量：

select 53 - 52 * 7 as a,
       53 - (52 * 7) as b,
       (53 - 52) * 7 as c
from dual;

    A     B     C
----- ----- -----
 -311  -311     7

当您使用 to_number() 时，您将超越默认优先级，但会进行不必要的隐式转换为字符串并显式转换回数字。仅括号就足以覆盖默认行为。

Answer 2

我想说我打赌这是一件 SQL/Plus 事情（Alex 证明了这一点，感谢 Alex）当我将它转换为 PL/SQL 匿名块并运行它时在 Toad 中，它按预期工作。我不打算 post 这个，因为它不是真正的答案，但是比较语言并注意可能会咬你的陷阱有点有趣，你应该记住，所以我会把它留到为了学习。

SET serveroutput ON

DECLARE
  today DATE;
  lastyear DATE;
  weekOfYearNow NUMBER;
  weekOfYearThen NUMBER;
  weekOfYearOffset NUMBER;
  lastYearFixed DATE;
  monday DATE;
  sunday DATE;
BEGIN
 today := TO_DATE('30/12/20', 'dd/mm/yy');
 lastYear := ADD_MONTHS(today, -12);

 DBMS_OUTPUT.PUT_LINE('Today: ' || today);
 DBMS_OUTPUT.PUT_LINE('LastYear: ' || lastYear);

 weekOfYearNow    := TO_NUMBER(TO_CHAR(today, 'ww'));
 weekOfYearThen   := TO_NUMBER(TO_CHAR(lastYear, 'ww'));
 weekOfYearOffset := weekOfYearNow - weekOfYearThen; -- <-- must use to_number - but why?

 lastYearFixed := lastYear + (weekOfYearOffset * 7); -- <-- this equals -311 if I don't call to_number above

 monday := ADD_MONTHS(SYSDATE - 7 - TO_CHAR(SYSDATE, 'd') + 2, -12);
 sunday := ADD_MONTHS(SYSDATE - 7 + TO_CHAR(SYSDATE, 'd'), -12);

 DBMS_OUTPUT.PUT_LINE('WeekOfYearNow: ' || WeekOfYearNow);
 DBMS_OUTPUT.PUT_LINE('WeekOfYearThen: ' || WeekOfYearThen);
 DBMS_OUTPUT.PUT_LINE('WeekOfYearOffset: ' || WeekOfYearOffset);
 DBMS_OUTPUT.PUT_LINE('LastYearFixed: ' || LastYearFixed);

END;

Today: 30-DEC-20
LastYear: 30-DEC-19
WeekOfYearNow: 53
WeekOfYearThen: 52
WeekOfYearOffset: 1
LastYearFixed: 06-JAN-20

数字乘以常数结果为负数

Number multiplied by constant results in a negative number

sql

oracle

operator-precedence

oracle-sqldeveloper