[英]SQL Server : create a column based on the first occurrence of a value in another column
考慮 SQL Server 中的下表:
我想編寫一個 SQL 查詢來生成列Indicator
。 對於每個類別,第一次出現Flag = 1
,應將此列設置為 1。
例如,對於類別 A,對於日期 1/3/2019、1/4/2019、1/5/2019、1/6/2019,列Flag
設置為 1。 由於 1/3/2019 是Flag
設置為 1 的最早日期,因此該記錄的Indicator
列也應設置為 1。
我應該為此編寫什么 SQL Server 查詢?
PS:圖中已經顯示了Indicator
列所需的輸出。
下面是在 SQL Server 中生成表的代碼:
CREATE TABLE myTable
(
Category CHAR(1),
Date DATE,
Flag INT
)
INSERT INTO myTable (Category, Date, Flag)
VALUES ('A', '2019-01-01', 0), ('A', '2019-02-01', 0),
('A', '2019-03-01', 1), ('A', '2019-04-01', 1),
('A', '2019-05-01', 1), ('A', '2019-06-01', 1),
('B', '2019-01-01', 0), ('B', '2019-02-01', 0),
('B', '2019-03-01', 0), ('B', '2019-04-01', 0),
('B', '2019-05-01', 1), ('B', '2019-06-01', 1),
('C', '2019-01-01', 0), ('C', '2019-02-01', 0),
('C', '2019-03-01', 0), ('C', '2019-04-01', 1),
('C', '2019-05-01', 1), ('C', '2019-06-01', 1),
('C', '2019-07-01', 1)
使用派生表和MIN()
確定具有標志的類別的第一個日期的一種方法。 將其連接回原始表。
SELECT
yt.*
, ISNULL(b.Indicator, 0) AS Indicator
FROM YourTable yt
LEFT JOIN
(SELECT category, MIN(date) AS date, 1 AS Indicator
FROM dbo.YourTable
WHERE Flag = 1
GROUP BY Category) b ON b.Category = yt.Category AND b.date = yt.date
我正在考慮使用min()
函數作為窗口函數:
select t.*,
(case then t.flag = 1 and
t.date = min(t.date) over (partition by t.category, t.flag)
then 1 else 0
end) as indicator
from myTable t
order by t.Category, t.date
其它的辦法
CREATE TABLE myTable
(
Category char(1),
Date date,
Flag int
)
INSERT INTO myTable (Category, Date, Flag) VALUES
('A','2019-01-01',0),
('A','2019-02-01',0),
('A','2019-03-01',1),
('A','2019-04-01',1),
('A','2019-05-01',1),
('A','2019-06-01',1),
('B','2019-01-01',0),
('B','2019-02-01',0),
('B','2019-03-01',0),
('B','2019-04-01',0),
('B','2019-05-01',1),
('B','2019-06-01',1),
('C','2019-01-01',0),
('C','2019-02-01',0),
('C','2019-03-01',0),
('C','2019-04-01',1),
('C','2019-05-01',1),
('C','2019-06-01',1),
('C','2019-07-01',1);
select t.* ,
CASE WHEN T.FLAG=1 AND FIRST_VALUE(T.DATE) OVER (PARTITION BY T.Category ORDER BY t.FLAG desc, t.Date asc)=T.DATE THEN 1
ELSE 0 END Indicator
from myTable t
order by t.Category, t.date
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