If you only want to match "" as an empty string
WHERE DATALENGTH(COLUMN) > 0
If you want to count any string consisting entirely of spaces as empty
WHERE COLUMN <> ''
Both of these will not return NULL values when used in a WHERE clause. As NULL will evaluate as UNKNOWN for these rather than TRUE.
CREATE TABLE T
(
C VARCHAR(10)
);
INSERT INTO T
VALUES ('A'),
(''),
(' '),
(NULL);
SELECT *
FROM T
WHERE C <> ''
Returns just the single row A. I.e. The rows with NULL or an empty string or a string consisting entirely of spaces are all excluded by this query.
SQL Fiddle
Answer from Martin Smith on Stack OverflowIf you only want to match "" as an empty string
WHERE DATALENGTH(COLUMN) > 0
If you want to count any string consisting entirely of spaces as empty
WHERE COLUMN <> ''
Both of these will not return NULL values when used in a WHERE clause. As NULL will evaluate as UNKNOWN for these rather than TRUE.
CREATE TABLE T
(
C VARCHAR(10)
);
INSERT INTO T
VALUES ('A'),
(''),
(' '),
(NULL);
SELECT *
FROM T
WHERE C <> ''
Returns just the single row A. I.e. The rows with NULL or an empty string or a string consisting entirely of spaces are all excluded by this query.
SQL Fiddle
WHERE NULLIF(your_column, '') IS NOT NULL
Nowadays (4.5 years on), to make it easier for a human to read, I would just use
WHERE your_column <> ''
While there is a temptation to make the null check explicit...
WHERE your_column <> ''
AND your_column IS NOT NULL
...as @Martin Smith demonstrates in the accepted answer, it doesn't really add anything (and I personally shun SQL nulls entirely nowadays, so it wouldn't apply to me anyway!).
Videos
I have a table which contains some values which are not null. I thought it might be an empty sting but it was not an empty string either.
I copied that particular field from the table and tried an update statement to convert it into null but it didn't worked either . Kindly suggest something
I would say that NULL is the correct choice for "no email address". There are many "invalid" email addresses, and "" (empty string) is just one. For example "foo" is not a valid email address, "a@b@c" is not valid and so on. So just because "" is not a valid email address is no reason to use it as the "no email address" value.
I think you're right in saying that "" is not the correct way to say "I don't have a value for this column". "" is a value.
An example of where "" might be a valid value, separate to NULL could be a person's middle name. Not every one has a middle name, so you need to differentiate between "no middle name" ("" - empty string) and "I don't know if this person has a middle name or not" (NULL). There's probably many other examples where an empty string is still a valid value for a column.
While agreeing with the above comments, I would add this argument as a primary motivation:
- It is obvious to any programmer looking at a database that a field marked NULL is an Optional field. (i.e. the record doesn't require data for that column)
- If you mark a field NOT NULL, any programmer should intuitively assume that it is a Required field.
- In a field that allows nulls, programmers should expect to see nulls rather than empty strings.
For the sake of Self-Documenting Intuitive Coding, use NULL instead of empty strings.
Let's say that the record comes from a form to gather name and address information. Line 2 of the address will typically be blank if the user doesn't live in apartment. An empty string in this case is perfectly valid. I tend to prefer to use NULL to mean that the value is unknown or not given.
I don't believe the physical storage difference is worth worrying about in practice. As database administrators, we have much bigger fish to fry!
I do not know about MySQL and PostgreSQL, but let me treat this a bit generally.
There is one DBMS namely Oracle which doesn't allow to choose it's users between NULL and ''. This clearly demonstrates that it is not necessary to distinguish between both. There are some annoying consequences:
You set a varchar2 to an empty string like this:
Update mytable set varchar_col = '';
the following leads to the same result
Update mytable set varchar_col = NULL;
But to select the columns where the value is empty or NULL, you have to use
select * from mytable where varchar_col is NULL;
Using
select * from mytable where varchar_col = '';
is syntactically correct, but it never returns a row.
On the other side, when concatenating strings in Oracle. NULL varchars are treated as empty strings.
select NULL || 'abc' from DUAL;
yields abc. Other DBMS would return NULL in these cases.
When you want to express explicitly, that a value is assigned, you have to use something like ' '.
And you have to worry whether trimming not empty results in NULL
select case when ltrim(' ') is null then 'null' else 'not null' end from dual
It does.
Now looking at DBMS where '' is not identical to NULL (e.g. SQL-Server)
Working with '' is generally easier and in most case there is no practical need to distinguish between both. One of the exceptions I know, is when your column represents some setting and you have not empty defaults for them. When you can distinguish between '' and NULL you are able to express that your setting is empty and avoid that the default applies.
I hate empty strings.
That being said, disallowing empty string as a policy doesn't solve the underlying problem; which is "garbage in=garbage out".
If I'm a developer who knows what I'm not allowed to use but I think it's stupid, I'll find an exception.
- no empty string? I'll insert
'x' - no single character? I'll insert
'xx' - some regex filter abomination? random text algorithms are cheap.
Disallow empty string (or other patterns) if you have client issues with garbage data. But whenever you're setting procedural rules to pre filter crap, make sure you reduce the demand as well as the supply. Figure out why people are demanding crap data and give them an alternative instead of fighting them at the tap.
Alternately you could use an additive check constraint or a custom data type to enforce certain input constraints
If I'm expecting the value of the field to be not null, shouldn't I also always have a constraint that does not allow for zero length strings or space only strings
Yes. Yes you should.
It's ANSI standard behavior to enable storing empty strings, but that doesn't mean it's a good idea. And you especially should not allow both Null and '' in a column.
And note that because strings are compared without trailing spaces,
check (SomeCol<>'')
suffices for both.
Select *
From Table
Where (col is null or col = '')
Or
Select *
From Table
Where IsNull(col, '') = ''
If you need it in SELECT section can use like this.
SELECT ct.ID,
ISNULL(NULLIF(ct.LaunchDate, ''), null) [LaunchDate]
FROM [dbo].[CustomerTable] ct
You can replace the null with your substitution value.