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MQTT 之保留消息（Retained Messages） - 创客硬件开发 - 清泛IT社区，为创新赋能！
保留消息定义 如果PUBLISH消息的RETAIN标记位被设置为1，则称该消息为“保留消息”；Broker对保留消息的处理 Broker会存储每个Topic的最后一条保留消息及其Qos，当订阅该Topic的客户端上线后，Broker需要将该消息投递给它。A r...
How to use DISTINCT and ORDER BY in same SELECT statement?
..., and use a GROUP BY to make the DISTINCT work.
Try something like this:
SELECT DISTINCT Category, MAX(CreationDate)
FROM MonitoringJob
GROUP BY Category
ORDER BY MAX(CreationDate) DESC, Category
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With MySQL, how can I generate a column containing the record index in a table?
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You may want to try the following:
SELECT l.position,
l.username,
l.score,
@curRow := @curRow + 1 AS row_number
FROM league_girl l
JOIN (SELECT @curRow := 0) r;
The JOIN (SELECT @curRow := 0) part allows the variable initiali...
Select n random rows from SQL Server table
I've got a SQL Server table with about 50,000 rows in it. I want to select about 5,000 of those rows at random. I've thought of a complicated way, creating a temp table with a "random number" column, copying my table into that, looping through the temp table and updating each row with RAND() , and ...
SQL SELECT WHERE field contains words
I need a select which would return results like this:
15 Answers
15
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How to Select Every Row Where Column Value is NOT Distinct
I need to run a select statement that returns all rows where the value of a column is not distinct (e.g. EmailAddress).
7 A...
How to select the nth row in a SQL database table?
I'm interested in learning some (ideally) database agnostic ways of selecting the n th row from a database table. It would also be interesting to see how this can be achieved using the native functionality of the following databases:
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How to reset postgres' primary key sequence when it falls out of sync?
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-- Login to psql and run the following
-- What is the result?
SELECT MAX(id) FROM your_table;
-- Then run...
-- This should be higher than the last result.
SELECT nextval('your_table_id_seq');
-- If it's not higher... run this set the sequence last to your highest id.
-- (wise to run...
SQL Server SELECT into existing table
I am trying to select some fields from one table and insert them into an existing table from a stored procedure. Here is what I am trying:
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How to write a foreach in SQL Server?
... the lines of a for-each, where I would like to take the Ids of a returned select statement and use each of them.
10 Answer...