Question

我想加入表A和表B，并显示每辆卡车的最新条目。

表A列：TA ID，Truck_Num，Serial_Num
表B列：TB ID，Serial_Num，Lat，Long，DateTime

期望的输出

 Truck   |     DateTime     |    Serial_Num   |       Lat   |      Long                
   1     |     datestamp    |       123       |        1    |       -1                                     
   2     |     datestamp    |       456       |        2    |       -2

使用联接查询

SELECT a.Truck b.Lat b.Long b.DateTime FROM TB as b INNER JOIN TA AS a a.Serial_Num=b.Serial_Num;

我试过这个，但它只给了我最近的参赛作品时间。

SELECT a.Truck b.Lat b.Long b.DateTime FROM TB as b INNER JOIN TA AS a a.Serial_Num=b.Serial_Num WHERE b.DateTime = (SELECT MAX(b.DateTime) FROM TB tb WHERE a.Truck_Num=Truck_Num);

Answer 1

这将显示每辆卡车的最新项目。如果要显示的其他信息在所有行中相同，则可以将其放在group by子句中，否则则必须使用此结果进行连接

SELECT Serial_Num, MAX(DateTime) FROM TB GROUP BY Serial_Num;

如果数据不相等且您无法将其放在group by子句上，那么这是您必须要进行的连接

SELECT Truck_Num, Lat, Long, maxDateTime FROM TB as b 
   INNER JOIN TA AS a ON a.Serial_Num=b.Serial_Num
   INNER JOIN (SELECT Serial_Num, MAX(DateTime) as maxDateTime
               FROM TB GROUP BY Serial_Num) as c 
               ON b.Serial_Num=c.Serial_Num AND maxDateTime = DateTime

Answer 2

它应该有效，你可以查看这个问题： Select only one row with max field with One-Many Relationship Table

SELECT
    ta.Truck,
    tb.Lat,
    tb.Long,
    MAX(tb.DateTime),
    ta.Serial_Num as Serial_Num
FROM
    TB as tb
INNER JOIN
    TA as ta
ON (tb.Serial_Num = ta.Serial_Num)
GROUP BY
    tb.Serial_Num

具有内连接的2个表的mysql查询

2 个答案: