问题

How do you handle this situation where Oracle stores the empty string as a null in the database ?

I would like it to be stored as an empty string as it is not as NULL, since issuing the query would be easier.

Something like this would select the empty string and non-empty string, but not the null values

select * from mytable where myfield like '%';

if i would like to select also the null values (which should be originally empty string), i would have to select like this :

select * from mytable where myfield like '%' or myfield is null;

i would love to skip doing or myfield is null all the time later in my sql statements

The current solution i have in mind is to take care of this in the application level, for example, in the entity, i initialize all my String field default value to a space, for example :

@Entity
public class MyEntity {
  private String name = " ";

  public void setName(String name) {
    if (isEmptyString(name)) {
      name = " ";
    }
  }
  ...

}

Or perhaps, i can make use of a new type still unknown to me from Oracle 11g that can keep empty string as it is without changing it to null value ?

Thank you !

回答1:

Yup, that's the way Oracle functions. Empty strings are treated as nulls.

You can of course "fix" this on application level - for example by storing " " values as you suggested - but first consider, what exactly is the difference with your "empty string" values compared to NULL values? Why do you need to treat them differently? I used to run into this dilemma, too, but usually found out that there are very few cases where I really need to tell the difference.

回答2:

It´s not only the selection with special where condition but also the treating of Java String Objects. If you have a String a="" you can call its length method and get 0. If you have a String a=null you get a nullpointer exception when calling length. So working with an oracle db forces you to always check if your string is null before checking length :(

回答3:

try

create index idx_myfield on mytable(nvl(myfield,-1));

select * from mytable where nvl(myfield,-1)=-1;

回答4:

No, there is no way to treat empty strings as empty strings. Oracle always treats a string of length zero as a NULL value.

回答5:

Its early for me, but isn't

select * from mytable where myfield like '%' or myfield is null

the same as

select * from mytable

So, Oracle simplifies your life! ;)

回答6:

Beleive Oracle is optimizing the database by converting the empty string as NULL. First an empty string still may be need to be stored explicitly in the database storage at data block level (*1). Storing as NULL may reduce data storage footprint. Second if any indexes were defined on the column then the NULL values are not included in index thereby reducing index storage footprint. (*2)

From a design and development standpoint unless the empty space really changes the semantics of the data from a business perspective storing as NULL should be fine.

Adding code at framework level ( or parent class ) as suggested above would eliminate the need to type it out at all children classes & objects - that is what Object Oriented and even basic programming strive to do.

If my code is performing best because my database storage is optimized, then I should be happy. It sucks if my code performance tanks in production just because I wanted to save a little typing or failed to do masterful design / development?

I would be happy that Oracle is optimizing it for me under the covers.