使用兩列具有相同的外鍵

Question

我有一個數據庫，其中有5個表，但我有問題。在裝運表中，有兩列，即來源和目的地，它們都是對路線表的外鍵引用，但是當我從裝運表中選擇一條記錄時，它只會爲兩者顯示相同的路線名。

下面是代碼：

SELECT fare, commission, driver,shipment._date, routename, vehiclenumber, productname, source, destination, routename,ownername 
FROM route, shipment, product, vehicle,owner 
WHERE vehicle.vehicleid = shipment.vehicle 
AND shipment.source 
and vehicle.owner=owner.ownerid 
AND shipment.destination = route.routeid 
AND shipment.product = product.productid 
AND vehicle.vehiclenumber = 'nk-234' 
ORDER BY _date 
LIMIT 0 , 30 

Answer 1

要在一個表（一個貨）在另一臺連接到記錄多個記錄（兩條路線），你會非常反對route表使用明確JOIN小號兩次（或更多，因爲你碰巧需要很多鏈接）。

下面是對您的查詢進行快速修改以演示使用情況。要特別注意在SELECT兩個routename列和兩個JOIN S到route表中FROM：

SELECT fare, 
     commission, 
     driver, 
     shipment._date, 
     RS.routename, <-- field from first join 
     vehiclenumber, 
     productname, 
     source, 
     destination, 
     RD.routename, <-- field from second join 
     ownername 

FROM shipment 
JOIN route RS ON RS.routeID = shipment.source  <-- join 1; source 
JOIN route RD ON RD.routeID = shipment.destination, <-- join 2; destination 
    product, 
    vehicle, 
    owner 

WHERE vehicle.vehicleid = shipment.vehicle 
    AND vehicle.owner = owner.ownerid 
    AND shipment.product = product.productid 
    AND vehicle.vehiclenumber = 'nk-234' 

ORDER BY _date 

LIMIT 0 , 30 

很明顯，這可能不完全符合您的需求，因爲我不知道是什麼的表其他選定字段從能夠確保它們都佔了...

Answer 2

這是因爲您從裝運一排的領域，ORER實際上從你的外鍵得到任何效果，你需要使用一些加入

http://dev.mysql.com/doc/refman/5.5/en/join.html

Answer 3

使用舊式（非首選）符號，你可以寫：

SELECT s.fare, s.commission, s.driver, s._date, v.vehiclenumber, p.productname, 
     s.source, s.destination, rs.routename, rd.routename, o.ownername 
    FROM route AS rs, route AS rd, shipment AS s, product AS p, vehicle AS v, owner AS o 
WHERE s.vehicleid = s.vehicle 
    AND s.source = rs.routeid 
    AND v.owner = o.ownerid 
    AND s.destination = rd.routeid 
    AND s.product = p.productid 
    AND v.vehiclenumber = 'nk-234' 
ORDER BY _date 
LIMIT 0, 30 

我不得不猜測哪些表包含票價，佣金和司機信息;我認爲他們都在發貨表中。

儘管大多數DBMS都允許，但在標準SQL中不允許使用下劃線開始列名。儘管如此，它看起來很醜陋。爲什麼不把'ship_date'作爲列名？

這是更好地使用顯式連接符號，不僅是因爲有較少忘記連接條件的機會：

SELECT s.fare, s.commission, s.driver, s._date, v.vehiclenumber, p.productname, 
     s.source, s.destination, rs.routename, rd.routename, o.ownername 
    FROM shipment AS s 
    JOIN route AS rs ON s.source  = rs.routeid 
    JOIN route AS rd ON s.destination = rs.routeid 
    JOIN vehicle AS v ON s.vehicleid = s.vehicle 
    JOIN owner AS o ON v.owner  = o.ownerid 
    JOIN product AS p ON s.product  = p.productid 
WHERE v.vehiclenumber = 'nk-234' 
ORDER BY s._date 
LIMIT 0, 30