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Simulate CREATE DATABASE IF NOT EXISTS for PostgreSQL?
...k around it from within psql by executing the DDL statement conditionally:
SELECT 'CREATE DATABASE mydb'
WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'mydb')\gexec
The manual:
\gexec
Sends the current query buffer to the server, then treats each column of each row of the query's outpu...
App Inventor 2 串口通信教程 - USB串口与蓝牙串口 · App Inventor 2 中文网
...机蓝牙已开启，并且已与 HC-05/HC-06 配对
HC-05 默认配对密码为 1234 或 0000
在 Android 系统设置中手动配对蓝牙设备后，再在 App 中连接
确保没有其他应用占用蓝牙连接
2. 串口打开失败（USB 模式）
问题描述...
CDN（内容分发网络）技术原理 - 更多技术 - 清泛网 - 专注C/C++及内核技术
...的整个过程。CDN网络是在用户和服务器之间增加Cache层，如何将用户的请求引导到Cache上获得源服务器的数据，主要是通过接管DNS实现，下面让我们看看访问使用CDN缓存后的网站的过程：
通过上图，我们可以了解到，使用了CDN...
Detecting when user has dismissed the soft keyboard
I have an EditText widget in my view. When the user selects the EditText widget, I display some instructions and the soft keyboard appears.
...
Stash only one file out of multiple files that have changed with Git?
...
You can also use git stash save -p "my commit message". This way you can select which hunks should be added to stash, whole files can be selected as well.
You'll be prompted with a few actions for each hunk:
y - stash this hunk
n - do not stash this hunk
q - quit; do not stash this hun...
App Inventor 2 中文网原创内容 · App Inventor 2 中文网
...科普】App Inventor 2 添加超过10个屏幕会怎样？
【算法】如何计算1加到100的总和？（经典循环的写法）
【算法】App Inventor 2 算法之二分算法（Binary Search）实现，快速查找定位
组件用法
【微数据库】每次...
AsyncProcedures异步过程扩展 · App Inventor 2 中文网
...作
常见问题
Q: 异步过程中如何更新UI？
Q: 如何处理异步过程的错误？
Q: 异步过程会阻塞应用吗？
Q: 如何控制异步过程的执行顺序？
相关扩展
版权信息
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How do Python functions handle the types of the parameters that you pass in?
...tively.
Iterable - useful for generators.
Any - when it could be anything.
Union - when it could be anything within a specified set of types, as opposed to Any.
Optional - when it might be None. Shorthand for Union[T, None].
TypeVar - used with generics.
Callable - used primarily for functions, but ...
count(*) vs count(column-name) - which is more correct? [duplicate]
...
there is a performance difference (at least in MySQL) as well (see my answer).
– nickf
Jun 9 '10 at 7:51
1
...
Computed / calculated / virtual / derived columns in PostgreSQL
...y as column?
The expression (looking like a column) is not included in a SELECT * FROM tbl, though. You always have to list it explicitly.
Can also be supported with a matching expression index - provided the function is IMMUTABLE. Like:
CREATE FUNCTION col(tbl) ... AS ... -- your computed expr...