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Remove columns from dataframe where ALL values are NA
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dplyr now has a select_if verb that may be helpful here:
library(dplyr)
temp <- data.frame(x = 1:5, y = c(1,2,NA,4, 5), z = rep(NA, 5))
not_all_na <- function(x) any(!is.na(x))
not_any_na <- function(x) all(!is.na(x))
> temp
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When and why are database joins expensive?
...s back to mitigating the size of the working set. Joins involving properly selected keys with correctly set up indexes are cheap, not expensive, because they allow significant pruning of the result before the rows are materialised.
Materialising the result involves bulk disk reads which are the mo...
How can I join multiple SQL tables using the IDs?
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You want something more like this:
SELECT TableA.*, TableB.*, TableC.*, TableD.*
FROM TableA
JOIN TableB
ON TableB.aID = TableA.aID
JOIN TableC
ON TableC.cID = TableB.cID
JOIN TableD
ON TableD.dID = TableA.dID
WHERE DATE(Tab...
How to select .NET 4.5.2 as a target framework in Visual Studio
... },
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true,enableSnippets:true
});
}
});
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Using LINQ to concatenate strings
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Real example from my code:
return selected.Select(query => query.Name).Aggregate((a, b) => a + ", " + b);
A query is an object that has a Name property which is a string, and I want the names of all the queries on the selected list, separated by comma...
Visual Studio refuses to forget breakpoints?
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Try this select the Breakpoints toolbar item from the Debug toolbar. This brings up the Breakpoints window. From this window, you can view all breakpoints in the application. Here, you select and clear the breakpoint by clicking the D...
How to find the size of an array in postgresql
...ikely) and are running PostgreSQL 9.4 or higher, you can use cardinality:
SELECT cardinality(id) FROM example;
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Is there a JSON equivalent of XQuery/XPath?
...g
tools including filtering, recursive search, sorting, mapping, range
selection, and flexible expressions with wildcard string comparisons
and various operators.
JSONselect has another point of view on the question (CSS selector-like, rather than XPath) and has a JavaScript implementation.
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Get day of week in SQL Server 2005/2008
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Use DATENAME or DATEPART:
SELECT DATENAME(dw,GETDATE()) -- Friday
SELECT DATEPART(dw,GETDATE()) -- 6
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getting type T from IEnumerable
... t.GetGenericTypeDefinition() == typeof(IEnumerable<>))
.Select(t => t.GetGenericArguments()[0]);
}
Some objects implement more than one generic IEnumerable so it is necessary to return an enumeration of them.
Edit: Although, I have to say, it's a terrible idea for a class to...