SQL查詢根據第一個表的輸入從第二個表中獲取字段

Question

我有 2 個表，table1 和 table2

表格1 -

錫	時間戳
123	123456

表2 -

錫	時間戳	代碼
456	123456	xxxxx

我想首先使用最大時間戳（紀元）從 table1 中獲取 sn，然后使用從 table1 返回的 sn 查詢表 2，並從 table2 中獲取具有最大時間戳的代碼。

我正在嘗試這個但遇到錯誤 -

select code from table2 where sn = (select sn, max(timestamp) from table1 GROUP BY sn)

我應該改用聯接嗎？

Answer 1

嘗試這個：

select DISTINCT code from table2 
where sn = (select sn from table1 WHERE timestamp = (SELECT MAX(timestamp) FROM Table1));

Answer 2

您可以使用以下代碼執行此操作：

SELECT code
from table2 t
where sn = (SELECT sn
            FROM table1 t1
            INNER JOIN (SELECT MAX(timestamp) AS max_ts
                        FROM table1) d
              ON t1.timestamp = d.max_ts
           )
INNER JOIN (SELECT MAX(timestamp) AS max_ts
           FROM table2) t2
  ON t.timestamp= t2.max_ts;

但是，使用您提供的示例表將不會返回任何結果，因為在 table2 中找不到 sn 123

Answer 3

為了獲得“最新”值，我總是使用窗口函數。

為了保持一切干凈，我使用了兩個 CTE 來定義兩個數據集，但如果需要，您可以將所有內容放在一個查詢中。

WITH latestDataT1 AS (
SELECT 
     sn
    ,timestamp
FROM (
    SELECT
        sn
        ,timestamp
        ,ROW_NUMBER() OVER(ORDER BY timestamp DESC) as RowNo
    FROM table1
)
WHERE
    RowNo = 1
)
WITH latestDataT2 AS (
SELECT
     sn
    ,timestamp
    ,code
FROM (
    SELECT 
         sn
        ,timestamp
        ,code
        ,ROW_NUMBER() OVER(PARTITION sn BY ORDER BY timestamp DESC) as RowNo
    FROM table2
)
WHERE
    RowNo = 1
)

SELECT
    *
FROM latestDataT1 AS t1
LEFT JOIN latestDataT2 AS t2
    ON t1.sn = t2.sn

Answer 4

您可以嘗試以下腳本之一：

1) select code from table2 where (sn, timestamp) in (select sn, max(timestamp) from table1 GROUP BY sn);

2)

select A.code, B.maxtime 
  from table2 A,  
       (select sn, max(timestamp) as maxtime 
          from table1 GROUP BY sn) B
 where A.sn        = B.sn
   and A.timestamp = B.maxtime;

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