mysql 时间比较 两种比较为什么有时候一样有时候不一样？

Question

SELECT
sum(amount)
FROM
order
WHERE 1 = 1
AND commit_time >= "2017-01-01 00:00:00"
AND commit_time <= "2017-01-31 23:59:59"

和
SELECT
sum(amount)
FROM
order
WHERE 1 = 1
and SUBSTR(commit_time,1,10)>= "2017-01-01 00:00:00"
and SUBSTR(commit_time,1,10)<= "2017-01-31 23:59:59"

Answer 1

数据不多，感觉相差不大，只不过是你后面这语句多用了SUBSTR函数而已。
如果你想再优化的话，可以把commit-time >=" 2017-01-01 00:00:00 " 改成（commit-time >" 2017-01-01 00:00:00 " or commit-time =" 2017-01-01 00:00:00 " ）会更快一点。