Use if exists in postgresql [closed]

Question

I know about the exists feature, but that does not help me with the given situation. What I want is:

Check if some row exists, and if exists then check for an another row if row exists.

Here's the sqlFiddle.

Description:

A location table:

| ID |   NAME | ADDRESS |
|----|--------|---------|
|  1 | India1 |  (null) |
|  2 | India2 |  (null) |
|  3 | India3 |  (null) |
|  4 | India4 |  (null) |
|  5 | India5 |  (null) |

Then a location_flag table:

| ID | LOCATION_ID | FLAG_ID | VALUE | PARENT_ID | DELETED |
|----|-------------|---------|-------|-----------|---------|
|  1 |           1 |       1 |   YES |    (null) |  (null) |
|  2 |           1 |       2 |   YES |    (null) |  (null) |
|  3 |           2 |       1 |   YES |    (null) |  (null) |
|  4 |           2 |       2 |    NO |    (null) |  (null) |

What I want is to get all locations except location with id=2.

In procedure, it'd be like:
if(locationWithFlag1='YES'){
 if(locationWithFlag2='YES'){
//make this row to be appear in result
}
}
else{
//make this row appear in result
}

Is it possible without bringing in plpgsql?

Answer 1

select l.* 
from location as l                             --- find all locations
where not exists                               --- where there isn't 
      ( select * 
        from location_flag as lf               --- a flag
        where lf.location_id = l.id 
          and lf.flag_id = 1                   --- with 1
          and lf.value = 'YES'                 --- and YES
          and not exists                       --- without 
              ( select *                      
                from location_flag as lf2      --- another flag
                where lf2.location_id = l.id 
                  and lf2.flag_id = 2          --- with 2
                  and lf2.value = 'YES'        --- and YES
              )
      ) ;

or:

select l.* 
from location as l 
where not exists 
      ( select * 
        from location_flag as lf
        where lf.location_id = l.id 
          and lf.flag_id = 1 
          and lf.value = 'YES'
      )
   or exists
      ( select * 
        from location_flag as lf2
        where lf2.location_id = l.id 
        and lf2.flag_id = 2 
        and lf2.value = 'YES'
      ) ;