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What is a Java ClassLoader?
...ed programming languages such as Java. In Java, the .class files generated by the Java compiler remain as-is until loaded into the Java Virtual Machine (JVM) -- in other words, the linking process is performed by the JVM at runtime. Classes are loaded into the JVM on an 'as needed' basis. And when a...
Delete duplicate records in SQL Server?
...
You can do this with window functions. It will order the dupes by empId, and delete all but the first one.
delete x from (
select *, rn=row_number() over (partition by EmployeeName order by empId)
from Employee
) x
where rn > 1;
Run it as a select to see what would be deleted:...
Summarizing multiple columns with dplyr? [duplicate]
...ackage contains summarise_all for this aim:
library(dplyr)
df %>% group_by(grp) %>% summarise_all(list(mean))
#> # A tibble: 3 x 5
#> grp a b c d
#> <int> <dbl> <dbl> <dbl> <dbl>
#> 1 1 3.08 2.98 2.98 2.91
#> 2 2 3...
How to count occurrences of a column value efficiently in SQL?
...
This should work:
SELECT age, count(age)
FROM Students
GROUP by age
If you need the id as well you could include the above as a sub query like so:
SELECT S.id, S.age, C.cnt
FROM Students S
INNER JOIN (SELECT age, count(age) as cnt
FROM Students
...
Is there a wikipedia API just for retrieve content summary?
... website, the flagship site of the Stack Exchange Network, created in 2008 by Jeff Atwood and Joel Spolsky, as a more open alternative to earlier Q&A sites such as Experts Exchange. The name for the website was chosen by voting in April 2008 by readers of Coding Horror, Atwood's popular programm...
Time complexity of Euclid's Algorithm
...al: a == b
Now we'll show that every single case decreases the total a+b by at least a quarter:
Tiny A: b % (a % b) < a and 2a <= b, so b is decreased by at least half, so a+b decreased by at least 25%
Tiny B: a % b < b and 2b <= a, so a is decreased by at least half, so a+b decrease...
Why are these numbers not equal?
...l places:
The R FAQ has question devoted to it: R FAQ 7.31
The R Inferno by Patrick Burns devotes the first "Circle" to this problem (starting on page 9)
David Goldberg, "What Every Computer Scientist Should Know About Floating-point Arithmetic," ACM Computing Surveys 23, 1 (1991-03), 5-48 doi>...
Lock, mutex, semaphore… what's the difference?
...rocesses.
A mutex is the same as a lock but it can be system wide (shared by multiple processes).
A semaphore does the same as a mutex but allows x number of threads to enter, this can be used for example to limit the number of cpu, io or ram intensive tasks running at the same time.
For a more d...
How do I combine two data-frames based on two columns? [duplicate]
...
See the documentation on ?merge, which states:
By default the data frames are merged on the columns with names they both have,
but separate specifications of the columns can be given by by.x and by.y.
This clearly implies that merge will merge data frames based on mor...
oracle group 取每组第一条 - 数据库(内核) - 清泛网 - 专注C/C++及内核技术
...记录：
方法一：
select type,min(code) from group_info group by type;
注意：select 后面的列要在group by 子句中，或是用聚合函数包含，否则会有语法错误。
方法二：
SELECT * FROM(
SELECT z.type , z.code ,ROW_NUMBER()
OVER(PARTITION BY z.type ORDE...