Should You Use \"IN\" or \"=\" for Single Values in MySQL Queries?

MySQL Performance: "IN" Clause vs. Equals Sign Operator for Single Values Revisited

In MySQL, the performance implications of using the "IN" clause versus the equals sign operator for single values have been a subject of debate. While some assert that it makes no difference, others suggest that checking the count of values before choosing the appropriate operator could yield performance benefits.

Comparing "IN" and "=" for Single Values

This question explores the potential performance differences between using the "IN" clause and the equals sign operator for a single value. The scenario involves a dynamic SQL statement built in PHP, where the array $object_ids holds numeric values representing a single value:

$sql = 'SELECT * FROM `users` WHERE `id` IN(' . implode(', ', $object_ids) . ')';

An alternative option would be to check the count of $object_ids before deciding which operator to use:

if(count($object_ids) == 1) {
    $sql = 'SELECT * FROM `users` WHERE `id` = ' . array_shift($object_ids);
} else {
    $sql = 'SELECT * FROM `users` WHERE `id` IN(' . implode(', ', $object_ids) . ')';
}

Empirical Evidence

To assess the performance implications, an EXPLAIN analysis was conducted on three similar queries: one with "= 1", one with "IN(1)", and one with "IN(1,2,3)". The results revealed that:

• MySQL optimizes "IN(1)" to be equivalent to "= 1" for this type of query.

Conclusion

Based on the EXPLAIN results, the performance penalty of checking the count of values appears to be outweighed by the optimization MySQL performs for "IN(1)". Therefore, for simple queries with a single value, it is generally recommended to use the "IN" clause rather than the equals sign operator.

However, it is always advisable to consult the specific documentation for MySQL and consider the complexity of the query before making a final decision.

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