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ALTER TABLE to add a composite primary key
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They both enforce uniqueness on that set of three fields, however from an indexing standpoint there is a difference. The fields are indexed from left to right.
For example, consider the following queries:
A) SELECT person, place, thing FROM provider WHERE person = 'foo' AND thing = 'bar';
B) SELE...
PostgreSQL: How to make “case-insensitive” query
...ny function) on the predicate columns--in this case "name"--will cause any indexes to no longer be seekable. If this is a large or frequently queried table, that could cause trouble. Case-insensitive collation, citext, or a function-based index will improve performance.
– Jorda...
Delete rows from a pandas DataFrame based on a conditional expression involving len(string) giving K
...df = df.drop(some labels)
df = df.drop(df[<some boolean condition>].index)
Example
To remove all rows where column 'score' is < 50:
df = df.drop(df[df.score < 50].index)
In place version (as pointed out in comments)
df.drop(df[df.score < 50].index, inplace=True)
Multiple condit...
Index on multiple columns in Ruby on Rails
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The order does matter in indexing.
Put the most selective field first, i.e. the field that narrows down the number of rows fastest.
The index will only be used insofar as you use its columns in sequence starting at the beginning. i.e. if you index ...
mysql实现split分割字符串（length, SUBSTRING_INDEX, substring） - 数据...
mysql实现split分割字符串（length, SUBSTRING_INDEX, substring）由于MySql没有直接的split函数，只提供了length, SUBSTRING_INDEX, substring三个函数，这里介绍如何用这三个函数实现split功能。# SUBS...由于MySql没有直接的split函数，只提供了length, SUB...
VIP会员中心 · App Inventor 2 中文网，少儿编程陪伴者
... Q & A 常见问题解答 App Inventor 2 中文网 和 电教馆的网站...
How to parse/format dates with LocalDateTime? (Java 8)
... Don't forget the uppercase on MM
– Wesos de Queso
Jun 25 '17 at 3:50
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Possible to iterate backwards through a foreach?
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When working with a list (direct indexing), you cannot do it as efficiently as using a for loop.
Edit: Which generally means, when you are able to use a for loop, it's likely the correct method for this task. Plus, for as much as foreach is implemented in-o...
Find and replace in file and overwrite file doesn't work, it empties the file
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When the shell sees > index.html in the command line it opens the file index.html for writing, wiping off all its previous contents.
To fix this you need to pass the -i option to sed to make the changes inline and create a backup of the original ...
How to sort two lists (which reference each other) in the exact same way
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The sorted index/map paradigm suggested by J.F. Sebastian is about 10% faster than either zip solution for me (using lists of 10000 random ints): %timeit index = range(len(l1)); index.sort(key=l1.__getitem__); map(l1.__getitem__, index...