>is an alias has to be defined ??

When you open a table with the USE command, the alias of the table is automatically defined to have the same name as the .dbf file upon which the table is based. For example,

USE mytable in 0

creates a table, with the alias name 'mytable' in a currently unused work area.

The USE command allows you to assign an alias which is different from the name of the table. Normally there is no reason to do this. But if you find it necessary to have the same .dbf file open in two separate workareas, having the ability to assign an alias name can be useful. For example.

USE mytable.dbf in 0
USE mytable.dbf AGAIN in 0 NOUPDATE ALIAS 'myreadonlytable'

opens the same table in two different work areas. The alias 'mytable' refers to the table that was opened up first, and the alias 'myreadonlytable' refers to the second copy of the table in a different work area.

>i used an alias in program to make a report and i've already cleared all tables
>in data environment
>but why my alias did'nt recognize, i used several aliases in my program..

I'm not sure why you expect the report generator to recognize the alias after the table has been cleared from the data environment.

An alias is the name that refers to a table open in a specific work area. (A good way to think about it is that the alias is the name of the work area containing the open table.) If the table is closed, the alias that was associated with it is no longer valid.

Hope this helps.