Empty IN clause parameter list in MySQL

问题

What happens when you do a SQL query where the IN clause is empty?

For example:

SELECT user WHERE id IN ();

Will MySQL handle this as expected (that is, always false), and if not, how can my application handle this case when building the IN clause dynamically?

回答1:

If I have an application where I'm building the IN list dynamically, and it might end up empty, what I sometimes do is initialize the list with an impossible value and add to that. E.g. if it's a list of usernames, I'll start with an empty string, since that's not a possible username. If it's an auto_increment ID, I'll use -1 because the actual values are always positive.

If this isn't feasible because there are no impossible values, you have to use a conditional to decide whether to include AND column IN ($values) expression in the WHERE clause.

回答2:

The closest approximation of this query with valid syntax is:

SELECT user FROM tbl1 WHERE id IN (SELECT id FROM tbl1 WHERE FALSE);

which unconditionally returns an empty result set. The subquery in the bracket always returns an empty set, and no value can be found in an empty set, since an empty set contains no values.

回答3:

This gives me 0 results as well:

SELECT id FROM User
WHERE id IN (NULL);

Tried on MYSQL 5.6

回答4:

If you use AUTO_INCREMENT for id (1,2,3, ..) and if array is empty, you can add one item [0]. So it will be

if (empty($arr)) {
   $arr[] = 0;
}

SELECT user WHERE id IN (0);

And there will be no mysql parse error. This case is very usefull in subqueries - when your main query is not dependent on subquery results.

Better way - don't call the query if array is empty.

$data = null;
if (!empty($arr)) {
    $data = ... call query
}

回答5:

Use an always false statement

Before creating the SQL, check for the array size. If the size is 0, generate the where statement as 1 = 2, as in:

SELECT * FROM USERS WHERE name in ()

becomes

SELECT * FROM USERS WHERE 1 = 2

For "not in" on an empty array, generate an always true statement as in:

SELECT * FROM USERS WHERE name not in ()

becomes

SELECT * FROM USERS WHERE 1 = 1

This should work for more complex queries as:

SELECT * FROM USERS WHERE (name in () OR name = 'Alice')

becomes

SELECT * FROM USERS WHERE (1 = 2 OR name = 'Alice')

回答6:

I assume that you still need to run the query when your IN is empty; for example with LEFT JOIN ... ON ... AND IN ().

As you are already dynamically building the IN in the application layer, I'd handle the empty case there.

Here's a basic example in PHP

$condition = 'FALSE' // Could feasibly want TRUE under different circumstances
if($values){
   $inParams = **dynamically generated IN parameters from $values**;
   $condition = 'IN (' . $inParams . ')';
}

$sql = 'SELECT * FROM `user` WHERE '.$condition;

If you don't need to run the query with an empty IN, then don't; save yourself a trip to the database!

N.B. In case you aren't already, I'd build/use a function that goes the long way round and binds in your IN parameters properly, rather than just concatting them raw into the SQL. This will give you some protection against SQL injection if it's used on raw data.

回答7:

Here is another variation of always false statement for empty lists that preserves both logic and the notion of actual column in the query.

Incorrect id in () can be rewritten into:

where id <> id;

Similarly incorrect negative condition id not in () can be transformed by custom query building code into:

where id = id;

This approach is safer than id not in (NULL) as it doesn't evaluate to NULL.

But beware that this would filter out of the result the rows where id is null. This may be considered a feature in some cases.

Espesially useful with complex stacked query building logic where nested builder is not aware how the resulting subquery could be used above.

回答8:

If you are using that query in an application and you pass dynamically a list of objects to the query, I should not call to the database to do a select with an impossible value, I should return an empty list without calling to the database query, directly.

Because it has no sense to do a query that you know that is empty before calling it.

来源：https://stackoverflow.com/questions/13210233/empty-in-clause-parameter-list-in-mysql

标签

mysql

sql

in-clause