如果第一个 SELECT 返回 0 行，则第二个 SELECT 查询

Second SELECT query if first SELECT returns 0 rows(如果第一个 SELECT 返回 0 行，则第二个 SELECT 查询)

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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