我假设您想知道以下内容之间的性能差异:

I assume you want to know the performance difference between the following:

WHERE foo IN ('a', 'b', 'c')
WHERE foo = 'a' OR foo = 'b' OR foo = 'c'

根据 MySQL 手册如果值是常量 IN 对列表进行排序，然后使用二进制搜索.我会想象 OR 没有特定的顺序一个一个地评估它们.所以 IN 在某些情况下更快.

According to the manual for MySQL if the values are constant IN sorts the list and then uses a binary search. I would imagine that OR evaluates them one by one in no particular order. So IN is faster in some circumstances.

***的了解方法是使用您的特定数据在您的数据库中对两者进行分析，以查看哪个更快.

The best way to know is to profile both on your database with your specific data to see which is faster.

我在一个有 1000000 行的 MySQL 上都试过.当该列被索引时，性能没有明显差异 - 两者几乎是即时的.当该列未编入索引时，我得到了以下结果:

I tried both on a MySQL with 1000000 rows. When the column is indexed there is no discernable difference in performance - both are nearly instant. When the column is not indexed I got these results:

SELECT COUNT(*) FROM t_inner WHERE val IN (1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000);
1 row fetched in 0.0032 (1.2679 seconds)

SELECT COUNT(*) FROM t_inner WHERE val = 1000 OR val = 2000 OR val = 3000 OR val = 4000 OR val = 5000 OR val = 6000 OR val = 7000 OR val = 8000 OR val = 9000;
1 row fetched in 0.0026 (1.7385 seconds)

所以在这种情况下，使用 OR 的方法慢了大约 30%.添加更多项会使差异更大.结果可能因其他数据库和其他数据而异.

So in this case the method using OR is about 30% slower. Adding more terms makes the difference larger. Results may vary on other databases and on other data.