TQuery.SQL中简单的SQL语句中一问。(30分)

S

springwater

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• 2002-04-15

• #1

SELECT Customer.CustNo, Customer.Company, Customer.City,
Customer.Country, Orders.OrderNo, Orders.AmountPaid, Employee.LastName,
Employee.FirstName, Employee.Salary, Employee.HireDate,
Employee.PhoneExt, Employee.EmpNo
FROM customer Customer
INNER JOIN [red]orders Orders[/red]
ON (Customer.CustNo = Orders.CustNo)
INNER JOIN employee Employee
ON (Orders.EmpNo = Employee.EmpNo)

李维先生的《Delphi5.X分布式多层应用系统篇》的多表更新一节中，一个
TQuery.SQL中写入了以上SQL语句，作用是连接了三张数据表（DBDEMOS数据
库中）。其中语法挺怪，"FROM","INNER JOIN"后跟的表名为什么要重复两遍
呢？这样写是什么意思呢？

 

E

epic

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• 2002-04-15

• #2

一个是表名，一个是别名，你注意到它前面select customen.custno,
就是指定了别名的结果，如果指定的别名是a，那么就可以写成select a.custno.

 

O

ohyee

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• 2002-04-15

• #3

别名啊

 

O

ourself

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• 2002-04-16

• #4

Agree with above 2 people:

别名常用于SQL Query，使用别名一般有两个好处：1.减少typing, (especially when
you want to specify Server name, database owner, etc); 2.便于读Code.但这里
别名显得太长，且别名与表名太Confusing， 所以你觉得语法挺怪,建议用较简单的别名.

An example as follows:

SELECT a.id1, a.name1
FROM table1 a INNER JOIN
table2 b ON a.id1 = b.id1

 

Y

yangxuhui

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• 2002-04-16

• #5

对，是别名

 

Y

Yong

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• 2002-04-16

• #6

to ourself :如果用a,b,c,d等别名，表多的时候容易搞混了

 

E

Expert

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• 2002-04-16

• #7

其实我倒喜欢用简单的别名，要不然不就失去了别名的意义了吗？
如：Sales S
Titles T
......

 

S

springwater

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• 2002-04-16

• #8

我初学，对别名不是很懂，请问各位,在上面的SQL语句中，并没有显示的定义：
比如：Customer表的别名为customer，是默认的吗？
如果我要把Customer表的别名指定为a，我又该怎么写,写在哪儿呢？
谢谢您!

 

O

ourself

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• 2002-04-16

• #9

1. To springwater:

Have a look at the following example. a is the alias (别名) of table1.

SELECT a.id1, a.name1
FROM table1 AS a INNER JOIN
table2 AS b ON a.id1 = b.id1

2. To Yong:

Agree with your. It's not a good idea to use a, b as 别名 havng too many tables.
I think we can use 别名 such as:
cpn (for company table),
psn (for person table),
cust (for customer table),etc.