SQL笛卡尔加入

Question

我正在尝试一个转换记录表和日期表。

表A：

date 
2017-01-01 
2017-01-02 
2017-01-03 
2017-01-04 

表B：

state | date_logged | requestno_i 
    A  2017-01-01  req01 
    B  2017-01-03  req01 
    C  2017-01-04  req01 

预期结果：

date  | state | date_logged | requestno_i 
2017-01-01  A  2017-01-01  req01 
2017-01-02  A  2017-01-01  req01 
2017-01-03  B  2017-01-03  req01 
2017-01-04  C  2017-01-04  req01 

这里是我的尝试：

SELECT 
A.date, 
B.state, 
B.date_logged, 
B.requestno_i 
FROM TABLE_A A CROSS JOIN TABLE_B B 
WHERE requestno_i='req01' and A.date>=B.date_logged 

Answer 1

这样做的一种方法是使用CROSS APPLY而不是CROSS JOIN。例如：

DECLARE @TableA TABLE ([Date] DATE); 
INSERT @TableA ([Date]) VALUES ('2017-01-01'), ('2017-01-02'), ('2017-01-03'), ('2017-01-04'); 

DECLARE @TableB TABLE ([State] CHAR(1), Date_Logged DATE, RequestNo_i VARCHAR(10)); 
INSERT @TableB ([State], Date_Logged, RequestNo_i) VALUES ('A', '2017-01-01', 'req01'), ('B', '2017-01-03', 'req01'), ('C', '2017-01-04', 'req01'); 

SELECT A.[Date], B.[State], B.Date_Logged, B.RequestNo_i 
FROM @TableA AS A 
CROSS APPLY 
(
    SELECT TOP 1 * 
    FROM @TableB 
    WHERE Date_Logged <= A.[Date] 
    AND RequestNo_i = 'req01' 
    ORDER BY Date_Logged DESC 
) AS B;