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set define [SQL*Plus]

set define on
set define off
set define x

set define x

set define x specifies the prefix-character for substitution variables. The default is the ampersand (&). 
set define +

select * from dba_objects 
 where object_name like '%+object_name%';

set define on

Turns on substitution variables. 
set define on

select '&hello' from dual;
If define is set to on and SQL*Plus finds the current substituion prefix, it asks for a string to be entered. In the following example, I entered: this string was entered 
Enter value for hello: this string was entered
old   1: select '&hello' from dual
new   1: select 'this string was entered' from dual

'THISSTRINGWASENTERED'
-----------------------
this string was entered
It might be annoying to see the following lines printed by SQL*Plus: 
old   1: select '&hello' from dual
new   1: select 'this string was entered' from dual
This behavior can be turned off by setting verify off.

set define off

Turns off substitution variables. 
set define off

select '&hello' from dual;

'&HELL
------
&hello

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See also define