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SQL keys, MUL vs PRI vs UNI
What is the difference between MUL , PRI and UNI in MySQL?
4 Answers
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How to concatenate columns in a Postgres SELECT?
... mean "returns NULL". The result of anything concatenated to NULL is NULL. If NULL values can be involved and the result shall not be NULL, use concat_ws() to concatenate any number of values (Postgres 9.1 or later):
SELECT concat_ws(', ', a, b) AS ab FROM foo;
Or concat() if you don't need separ...
Getting hold of the outer class object from the inner class object
... refer to any lexically enclosing instance, is described in the JLS as Qualified this.
I don't think there's a way to get the instance from outside the code of the inner class though. Of course, you can always introduce your own property:
public OuterClass getOuter() {
return OuterClass.this;
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What is the difference between '>' and a space in CSS selectors?
... other elements inbetween).
A B will select any B that are inside A, even if there are other elements between them.
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git: difference between “branchname” and “refs/heads/branchname”
...g you have all three types of refs in your repository.
refs/heads/0.58 specifies a branch named 0.58. If you don't specify what namespace the ref is in, git will look in the default ones. This makes using only 0.58 conceivably ambiguous - you could have both a branch and a tag named 0.58.
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SQL - many-to-many table primary key
...uffer, I'll stand by that comment (technically, it's a generalisation only if I say "all tables", "vast majority" is based on experience). Let's also think about your example, an order is created once (it may be updated occasionally but that's unlikely to change key/index info, more to hit things li...
What is the difference between Scope_Identity(), Identity(), @@Identity, and Ident_Current()?
...) all get the value of the identity column, but I would love to know the difference.
7 Answers
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How to add pandas data to an existing csv file?
I want to know if it is possible to use the pandas to_csv() function to add a dataframe to an existing csv file. The csv file has the same structure as the loaded data.
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Combine two or more columns in a dataframe into a new column with a new name
For example if I have this:
8 Answers
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How to combine paths in Java?
...you should use a class which is designed to represent a file system path.
If you're using Java 7 or Java 8, you should strongly consider using java.nio.file.Path; Path.resolve can be used to combine one path with another, or with a string. The Paths helper class is useful too. For example:
Path pa...