如何连接两个不同表的列

Question

i have two tables called overview and current_voltage. i want to get top 1 record of two different tables in a single table output.

sql query

    select top 1  VL1,VL2,VL3 from current_voltage
    where deviceimei ='233'
    order by devicetimestamp desc      
    union
    select top 1  OTI,WTI,ATI from overview
    where deviceimei ='233'
    order by devicetimestamp desc

req op

VL1,VL2,VL3,OTI,WTI,ATI
234,235,234,25,24,25

Answer 1

A simple JOIN would do. Here we go:

SELECT TOP 1 cv.VL1, cv.VL2, cv.VL3, ov.OTI, ov.WTI, ov.ATI
FROM current_voltage cv
JOIN overview ov
ON cv.deviceimei = ov.deviceimei
WHERE cv.deviceimei ='233'
ORDER BY cv.devicetimestamp DESC, ov.devicetimestamp DESC

To read more about JOIN in sql, refer this

Answer 2

You need a cross join of the 2 queries:

select t1.*, t2.*
from (    
    select top 1  VL1,VL2,VL3 from current_voltage
    where deviceimei ='233'
    order by devicetimestamp desc      
) t1 cross join (
    select top 1  OTI,WTI,ATI from overview
    where deviceimei ='233'
    order by devicetimestamp desc
) t2

Answer 3

You can also use lateral join:

select vc.*, ov.*
from current_voltage cv cross apply
     (select top (1) ov.*
      from overview ov
      where ov.deviceimei = cv.deviceimei 
      order by ov.devicetimestamp  desc
     ) ov 
where ov.deviceimei ='233'
order by cv.devicetimestamp  desc;