Question

我目前正在为本地站点进行一些嵌入式SQL查询。我陷入困惑。我需要这样做“向用户询问制造商。打印该制造商的所有产品的规格。即打印型号，产品类型以及适合该类型的任何关系的所有属性。”

我尝试了以下查询

(SELECT * FROM product NATURAL JOIN pc WHERE maker = '$m') 
union (SELECT * FROM product NATURAL JOIN printer WHERE maker = '$m') 
union (SELECT * FROM product NATURAL JOIN laptop WHERE maker = '$m');

但问题是每个表（个人电脑，笔记本电脑，打印机）没有相同的列。我认为如果表中没有包含该列，只需填写空值就可以解决我的问题。我试图将所有表和列合并到一个超级表中并将其显示给用户。然后我也想尝试按产品类型订购大表。我认为我只会在查询结束时给出一个ORDER BY。所以我的问题是如何加入这些表？

关系：

Product(maker, model, type (laptop, printer, pc))
Printer(model, color (0,1), type(ink-jet, laser), price)
Laptop(model, speed, ram, hd, screen, price)
Pc(model, speed, ram hd, price)

Answer 1

查看“SQL表别名”以使用较少的列向表中添加列。或者您可以输入空值，如：

Select Col1, Col2, Col3 from Table1
Union
Select Col1, Null as Col2, Null as Col3 from Table2

Answer 2

是的，您可以使用UNION来完成这项工作。只需使所有SELECT列表具有相同的名称和相同的类型，如下所示：

SELECT (a,b) FROM TABLE TableB NATURAL JOIN (

    SELECT a,b,c, NULL as d from TableC
    UNION
    SELECT a,b,NULL as c, d from TableD

) AS u ON u.b = TableB.b

Answer 3

您只需为不存在的字段选择NULL值。

(SELECT maker,pc.model,p.model,price,NULL AS color,speed,pc.type,pc.type,ram,hd,NULL AS screen
  FROM product p NATURAL JOIN pc WHERE maker = '$m') 
UNION 
(SELECT maker,printer.model,p.model,price,color,NULL AS speed,printer.type,p.type,NULL AS ram,NULL AS hd,NULL AS screen
  FROM product p NATURAL JOIN printer WHERE maker = '$m') 
UNION
(SELECT maker,laptop.model,p.model,price,NULL AS color,speed,laptop.type,laptop.type,ram,hd,screen
  FROM product p NATURAL JOIN laptop WHERE maker = '$m');

Answer 4

我认为你的意思是使用join关键字，而不是union关键字。

查看http://dev.mysql.com/doc/refman/5.0/en/join.html

连接具有diff列数的表

4 个答案: