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【BLE技术内幕】BLE技术揭秘 - 创客硬件开发 - 清泛IT论坛，有思想、有深度
...是移动智能终端，智能家居，互联设备等领域，主要特点包括：低功耗，使用纽扣电池就可以运行数月至数年。快连接，毫秒级的连接速度，传统蓝牙甚至长达数分钟。远距离，长达数百米的通信距离，而传统蓝牙通常10米左右...
How to add an auto-incrementing primary key to an existing table, in PostgreSQL?
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(Updated - Thanks to the people who commented)
Modern Versions of PostgreSQL
Suppose you have a table named test1, to which you want to add an auto-incrementing, primary-key id (surrogate) column. The following command should be sufficient in recent versions of PostgreSQL:
ALTER TABLE test1 A...
Checking for an empty field with MySQL
... and there's one record with null hotline but it doesn't match.I'm using MySQL 5.6
– Scott Chu
Apr 15 '16 at 10:16
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Adding multiple columns AFTER a specific column in MySQL
...mitted in a single ALTER TABLE statement, separated by commas. This is a MySQL extension to standard SQL, which permits only one of each clause per ALTER TABLE statement.
– informatik01
May 1 '19 at 13:21
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“INSERT IGNORE” vs “INSERT … ON DUPLICATE KEY UPDATE”
...ut the values you insert don't map to a partition.
If you use REPLACE, MySQL actually does a DELETE followed by an INSERT internally, which has some unexpected side effects:
A new auto-increment ID is allocated.
Dependent rows with foreign keys may be deleted (if you use cascading foreign keys...
What is SELF JOIN and when would you use it? [duplicate]
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SQL self-join simply is a normal join which is used to join a table to itself.
Example:
Select *
FROM Table t1, Table t2
WHERE t1.Id = t2.ID
share...
Handling warning for possible multiple enumeration of IEnumerable
...oom for too many assumptions. Do you really want someone to pass a LINQ to SQL query to this method, only for you to enumerate it twice (getting potentially different results each time?)
The semantic missing here is that a caller, who perhaps doesn't take time to read the details of the method, may...
How to move columns in a MySQL table?
Currently I am having the following MySQL table: Employees (empID, empName, department);
4 Answers
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MySQL Workbench: How to keep the connection alive
Error Code: 2013. Lost connection to MySQL server during query
9 Answers
9
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Rails: select unique values from a column
...iq.pluck(:rating) is the most efficient way of doing this - this generates SQL which use SELECT DISTINCT rather than applying .uniq to an array
– Mikey
Jun 19 '13 at 15:38
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