In SQL injection, why must the ID of union joint query be equal to 0

In fact, it is not necessary to let id=0

?id=0' union select 1,2,3 --+ ?id=0' union select 1,database(),3 --+
?id=0' union select 1,database(),user() --+

there id Special attention is needed ：

If there are only two echo positions

that id You must fill in a nonexistent number （-1 perhaps 0）, Joint query can echo successfully

Because in the data table ,id by 1,2,3,……

If the parameter is ：
?id=1' union select 1,2,3 --+

So the query statement is ：
select * from users where id='1' union select 1,2,3 --+' limit 0,1

id=1 The data is queried , Return to page , It occupies two echo positions

This is the time , union The content of the query , Because the only two echo bits are occupied , It makes it impossible to echo

If id=2, The returned content will also occupy the echo bit , Lead to union The content of the query cannot be echoed to the page

If id Is a number that does not exist in the table , for example id=0,id=-1 wait

So the query statement is ：
select * from users where id='0' union select 1,2,3 --+' limit 0,1

because id=0 No query results , So the page returns union The content of the query

You don't have to let id=0

Actually id=-1,id=-5 It's OK to wait

therefore , We can know ：

as long as union The previous query failed , that union The following query results can be echoed to the page

id=1’ and 1=2 union select 1,2,3 –+   adopt  and 1=2  Statement to trigger an error 
id=1’ or 1=1 union select 1,2,3 –+    adopt  or 1=1   Statement to trigger an error