从日期记录中获取日期作为列标题

Question

我在表EMP_DATES中有以下数据。

我想从表格中实现以下输出：

我该怎么做。我使用了以下 SQL 查询，但不确定如何将 Day 作为列标题。

SELECT TO_CHAR (start_date, 'Day'),
       TO_CHAR (completion_date, 'Day'),
       start_date,
       completion_date,
       ROUND ( (completion_date - start_date) * 24, 1)
  FROM emp_dates

表：

CREATE TABLE EMP_DATES
(
   START_DATE        DATE,
   COMPLETION_DATE   DATE
);


SET DEFINE OFF;

INSERT INTO EMP_DATES (START_DATE, COMPLETION_DATE)
     VALUES (
               TO_DATE ('02/29/2016 12:24:25', 'MM/DD/YYYY HH24:MI:SS'),
               TO_DATE ('02/29/2016 15:30:00', 'MM/DD/YYYY HH24:MI:SS'));

INSERT INTO EMP_DATES (START_DATE, COMPLETION_DATE)
     VALUES (
               TO_DATE ('03/01/2016 07:00:00', 'MM/DD/YYYY HH24:MI:SS'),
               TO_DATE ('03/01/2016 11:54:25', 'MM/DD/YYYY HH24:MI:SS'));

COMMIT;

Answer 1

您正在尝试将行转换为列，但由于您使用的是 10g，因此实际的 PIVOT 子句不可用；所以你需要用case语句和聚合的扩展方式来做：

SELECT
  SUM(CASE WHEN TO_CHAR (start_date, 'FMDay') = 'Sunday' THEN
    ROUND ( (completion_date - start_date) * 24, 1) END) AS "Sunday",
  SUM(CASE WHEN TO_CHAR (start_date, 'FMDay') = 'Monday' THEN
    ROUND ( (completion_date - start_date) * 24, 1) END) AS "Monday",
  SUM(CASE WHEN TO_CHAR (start_date, 'FMDay') = 'Tuesday' THEN
    ROUND ( (completion_date - start_date) * 24, 1) END) AS "Tuesday",
  SUM(CASE WHEN TO_CHAR (start_date, 'FMDay') = 'Wednesday' THEN
    ROUND ( (completion_date - start_date) * 24, 1) END) AS "Wednesday",
  SUM(CASE WHEN TO_CHAR (start_date, 'FMDay') = 'Thursday' THEN
    ROUND ( (completion_date - start_date) * 24, 1) END) AS "Thursday",
  SUM(CASE WHEN TO_CHAR (start_date, 'FMDay') = 'Friday' THEN
    ROUND ( (completion_date - start_date) * 24, 1) END) AS "Friday",
  SUM(CASE WHEN TO_CHAR (start_date, 'FMDay') = 'Saturday' THEN
    ROUND ( (completion_date - start_date) * 24, 1) END) AS "Saturday"
FROM emp_dates;

    Sunday     Monday    Tuesday  Wednesday   Thursday     Friday   Saturday
---------- ---------- ---------- ---------- ---------- ---------- ----------
                  3.1        4.9                                            

'FM' format modifier 阻止to_char() 产生的值被空格填充，这就是'Day' 和'Dy' 默认情况下发生的情况；否则，文本文字也需要填充，例如'Monday '.

您可以使用子查询来稍微简化代码：

SELECT
  SUM(CASE WHEN dy = 'Sun' THEN hours END) AS "Sunday",
  SUM(CASE WHEN dy = 'Mon' THEN hours END) AS "Monday",
  SUM(CASE WHEN dy = 'Tue' THEN hours END) AS "Tuesday",
  SUM(CASE WHEN dy = 'Wed' THEN hours END) AS "Wednesday",
  SUM(CASE WHEN dy = 'Thu' THEN hours END) AS "Thursday",
  SUM(CASE WHEN dy = 'Fri' THEN hours END) AS "Friday",
  SUM(CASE WHEN dy = 'Sat' THEN hours END) AS "Saturday"
FROM (
  SELECT TO_CHAR (start_date, 'FMDy', 'NLS_DATE_LANGUAGE=English') AS dy,
    ROUND ( (completion_date - start_date) * 24, 1) AS hours
  FROM emp_dates
);

    Sunday     Monday    Tuesday  Wednesday   Thursday     Friday   Saturday
---------- ---------- ---------- ---------- ---------- ---------- ----------
                  3.1        4.9                                            

我还更改了使用缩写的日期名称，因为它更短，并指定了要使用的语言 - 以防运行它的客户端会话不是英语，这将与字符串文字进行比较失败。 （您可以阅读更多关于 in the documentation 的信息）。

例如，在第一个版本中，如果会话语言是法语，to_char() 将得到“Lundi”，这与固定值“Monday”不匹配。您也可以使用天数进行比较，但这也受 NLS 设置的影响，并且无法像语言那样更改。