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postgresql - sql - count of `true` values
...
SELECT COALESCE(sum(CASE WHEN myCol THEN 1 ELSE 0 END),0) FROM <table name>
or, as you found out for yourself:
SELECT count(CASE WHEN myCol THEN 1 END) FROM <table name>
...
Delete sql rows where IDs do not have a match from another table
...HERE f.id IS NULL
Using NOT EXISTS:
DELETE FROM BLOB
WHERE NOT EXISTS(SELECT NULL
FROM FILES f
WHERE f.id = fileid)
Using NOT IN:
DELETE FROM BLOB
WHERE fileid NOT IN (SELECT f.id
FROM FILES f)
Warning
Whenever possible, per...
写出高质量代码的10个Tips - 更多技术 - 清泛网 - 专注C/C++及内核技术
写出高质量代码的10个Tips很长一段时间以来，我都在关注如何提高代码质量，也为此做过一些尝试，我想这个话题可能大家会比较感兴趣，在这里分享一下我关于如何提高代...很长一段时间以来，我都在关注如何提高代码质量，...
Edit the root commit in Git?
...t
Then the root commit will show up in the rebase TODO list, and you can select to edit or reword it:
reword <root commit sha> <original message>
pick <other commit sha> <message>
...
This is the explanation of --root from the Git rebase docs (emphasis mine):
Rebase ...
Converting an array of objects to ActiveRecord::Relation
...e clause will generate a SQL statement with IN that looks something like:
SELECT .... WHERE `my_models`.id IN (2, 3, 4, 6, ....
Keep in mind that the ordering of the array will be lost - But since your objective is only to run a class method on the collection of these objects, I assume it won't b...
What is sharding and why is it important?
... a DBMS for which the locality is quite restricted (say, a user only fires selects with a 'where username = $my_username') it makes sense to put all the usernames starting with A-M on one server and all from N-Z on the other. By this you get near linear scaling for some queries.
Long story short: S...
How do you uninstall MySQL from Mac OS X?
I accidentally installed the PowerPC version of MySQL on my Intel Mac in Snow Leopard, and it installed without a problem but of course doesn't run properly. I just didn't pay enough attention. Now when I try to install the correct x86 version it says that it can't install because a newer version is...
What are DDL and DML?
...als with data
manipulation, and includes most common SQL statements such SELECT,
INSERT, UPDATE, DELETE etc, and it is used to store, modify, retrieve,
delete and update data in database.
SELECT – retrieve data from the a database
INSERT – insert data into a table
UPDATE – up...
如何建立一套适合自己的高胜算交易系统 - 更多技术 - 清泛网 - 专注C/C++及内核技术
如何建立一套适合自己的高胜算交易系统这个交易系统是非机械的，适合你自己个性的，有完善的交易思想、细致的市场分析和整体操作方案的，在风险市场的赢家都有自已的交易系统，因...这个交易系统是非机械的，适合你自...
Variable declaration placement in C
...ly if the curlies are part of a block (not if they are part of a struct or union declaration or a braced initializer.)
– Jens
Jun 18 '13 at 19:39
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