大约有 40,000 项符合查询结果（耗时：0.0310秒） [XML]
Counting null and non-null values in a single query
...le and SQL Server (you might be able to get it to work on another RDBMS):
select sum(case when a is null then 1 else 0 end) count_nulls
, count(a) count_not_nulls
from us;
Or:
select count(*) - count(a), count(a) from us;
...
Is it possible to specify condition in Count()?
...an use the fact that the count aggregate only counts the non-null values:
select count(case Position when 'Manager' then 1 else null end)
from ...
You can also use the sum aggregate in a similar way:
select sum(case Position when 'Manager' then 1 else 0 end)
from ...
...
How to randomly select rows in SQL?
...
SELECT TOP 5 Id, Name FROM customerNames
ORDER BY NEWID()
That said, everybody seems to come to this page for the more general answer to your question:
Selecting a random row in SQL
Select a random row with MySQL:
SELECT...
MySQL root password change
I have been trying to reset my MySQL root password. I have run the mysqld_safe --skip-grant-tables, updated the root password, and checked the user table to make sure it is there. Once restarting the mysql daemon I tried logging in with the new root password that I just set and still get Access de...
Export and Import all MySQL databases at one time
I want to keep a backup of all my MySQL databases. I have more than 100 MySQL databases. I want to export all of them at the same time and again import all of them into my MySQL server at one time. How can I do that?
...
Return a value if no rows are found in Microsoft tSQL
...
SELECT CASE WHEN COUNT(1) > 0 THEN 1 ELSE 0 END AS [Value]
FROM Sites S
WHERE S.Id = @SiteId and S.Status = 1 AND
(S.WebUserId = @WebUserId OR S.AllowUploads = 1)
...
libevent对比libev的基准测试 - C/C++ - 清泛网 - 专注C/C++及内核技术
...libevent 每次迭代的总时间增加得快得多，无论客户端数量如何，花费的时间几乎是 libev 的两倍。不过，两者都表现出相似的增长特征。
轮询时间也非常相似，libevent 在 1000-fd 情况下始终较慢，而在 100-fd 情况下，时间几乎相同...
What is the PostgreSQL equivalent for ISNULL()
...
SELECT CASE WHEN field IS NULL THEN 'Empty' ELSE field END AS field_alias
Or more idiomatic:
SELECT coalesce(field, 'Empty') AS field_alias
share...
MongoDB: Combine data from multiple collections into one..how?
...
Doing unions in MongoDB in a 'SQL UNION' fashion is possible using aggregations along with lookups, in a single query. Here is an example I have tested that works with MongoDB 4.0:
// Create employees data for testing the union.
d...
初窥InnoDB的Memcached插件 - 大数据 & AI - 清泛网 - 专注C/C++及内核技术
...插件目录我们能看到innodb_engine.so和libmemcached.so：
mysql> SELECT @@plugin_dir;
+------------------------------+
| @@plugin_dir |
+------------------------------+
| /usr/local/mysql/lib/plugin/ |
+------------------------------+
此外还需要导入Memcached插...