问题描述
我正在尝试加快 PHP 脚本的运行速度，目前我正在该事物的 Mysql 域中推送一些 PHP 逻辑.如果第一个 Select 不返回行或计数为零，是否有办法进行不同的选择查询?
I am trying to speed up a PHP script and I am currently pushing some PHP logic in the Mysql domain of the thing. Is there a way to make a different select query if the first Select returns no rows, or a count of zero ?
请记住，第一个查询需要先运行，第二个查询应该仅在第一个查询返回空集时激活.
Keeping in mind that the first query needs to run first, and the second should only be activated if the first one returns an empty set.
SELECT * FROM proxies WHERE (A='B') || SELECT * FROM proxies WHERE (A='C')
对于上面的 2 个查询，我有这个代码，但它似乎每个查询运行两次(一次计数，一次返回).有没有更好的方法来做到这一点?
For the above 2 queries I have this code, but it seems to run each query twice (once to count, and once to return). Is there a better way to do this?
IF (SELECT count(*) FROM proxies WHERE A='B')>0
THEN SELECT * FROM proxies WHERE A='B'
ELSEIF (SELECT count(*) FROM proxies WHERE A='C')>0
THEN SELECT * FROM proxies WHERE A='C'
END IF
推荐答案
一种选择是将 UNION ALL 与 EXISTS 一起使用:
One option would be to use UNION ALL with EXISTS:
SELECT *
FROM proxies
WHERE A='B'
UNION ALL
SELECT *
FROM proxies
WHERE A='C' AND NOT EXISTS (
SELECT 1
FROM proxies
WHERE A='B'
)
- SQL Fiddle 演示
这将返回 proxies 表中的行，其中 A='B' 如果它们存在.但是，如果它们不存在，它将查找那些带有 A='C' 的行.
This will return rows from the proxies table where A='B' if they exist. However, if they don't exist, it will look for those rows with A='C'.
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