Question

我一直在接受一个我无法弄清楚的JOIN声明。

我想在两个不同的表中获取最新（最新）的条目，并将结果连接到输出中。

2个Select语句看起来像这样，它们按预期工作但当我尝试将它们作为JOIN语句重写时，我似乎无法得到正确的结果。

请帮助，谢谢。

SELECT MaxNum 
FROM table1 
WHERE UserID = 4
ORDER BY Date DESC
LIMIT 1

SELECT MinNum 
FROM table2 
WHERE UserID = 4
ORDER BY Date DESC
LIMIT 1

Answer 1

我不确定连接在这里是否有意义。但是，您可以稍微修改一下您当前的查询内容：

xaxis

Answer 2

由于每个查询只返回一行，您可以使用的一个脏技巧是cross join结果：

SELECT MaxNum, MinNum
FROM   (SELECT   MaxNum 
        FROM     table1 
        WHERE    UserID = 4
        ORDER BY Date DESC
        LIMIT    1) t 
CROSS JOIN (SELECT   MinNum 
            FROM     table2 
            WHERE    UserID = 4
            ORDER BY Date DESC
            LIMIT    1) s

Answer 3

如果要将结果显示为一行，请将查询用作FROM子句中的子查询。

SELECT *
FROM 
  (SELECT MaxNum FROM table1 WHERE UserID = 4 ORDER BY `Date` DESC LIMIT 1) AS q1,
  (SELECT MinNum FROM table2 WHERE UserID = 4 ORDER BY `Date` DESC LIMIT 1) AS q2;

Answer 4

select TOP 1 table1.MaxNum, table2.MinNum 
FROM table1 INNER JOIN table2 
ON table1.UserID = table2.UserID 
WHERE table1.UserID = 4 
ORDER BY table1.Date DESC

将2个select语句重写为一个JOIN语句

4 个答案: