MySQL中使用自定义变量 编写偷懒的UNION示例
以下是对MySQL中使用自定义变量,编写一个UNION的示例进行了详细的介绍,需要的朋友可以过来参考下 (参考自高性能MySQL) 假设有这样的需求:写一个UNION查询,其第一个子查询作为分支先执行,如果找到了匹配的行,则不再执行第二个分支的查询。 一般来说,我
以下是对MySQL中使用自定义变量,编写一个UNION的示例进行了详细的介绍,需要的朋友可以过来参考下
(参考自>)
假设有这样的需求:写一个UNION查询,其第一个子查询作为分支先执行,如果找到了匹配的行,,则不再执行第二个分支的查询。
一般来说,我们可以写出这样的UNION查询:
复制代码 代码如下:
select id from users where id=123456
union all
select id from users_archived where id = 123456;
此查询可以正常运行,但是无论在users表中是否找到记录,都会到users_archived表中扫描一次;因此可能也会返回重复的记录。为了减少这种情况下不必要的开销,SQL语句可以写成这样:
复制代码 代码如下:
SELECT GREATEST(@found := -1, id) AS id, 'users' AS which_tbl
FROM users WHERE id = 1
UNION ALL
SELECT id, 'users_archived'
FROM users_archived WHERE id = 1 and @found IS NULL
UNION ALL
SELECT 1, 'reset' FROM DUAL WHERE (@found := NULL) IS NOT NLL;
上 面的查询用到了自定义变量@found,通过在结果列中做一次赋值并且放在GREATEST函数中,以避免返回额外的数据。如果第一个分支查询结果集为 NULL,那@found自然也还是NULL,因此会执行第二个分支查询。另外,为了不影响后面的遍历结果,在查询的末尾将@found重置为NULL。
另外, 返回的第二列数据是为了说明这条记录是在users表还是在users_archived表中查询得到的。

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