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Sample random rows in dataframe
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The data.table package provides the function DT[sample(.N, M)], sampling M random rows from the data table DT.
library(data.table)
set.seed(10)
mtcars <- data.table(mtcars)
mtcars[sample(.N, 6)]
mpg cyl disp hp drat wt ...
Inserting a Python datetime.datetime object into MySQL
I have a date column in a MySQL table. I want to insert a datetime.datetime() object into this column. What should I be using in the execute statement?
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Get generated id after insert
... @GrAnd, but what if I'll delete some "start-middle" rows in my table, so I break the sequence of n-th rows with generated id=n. Will that returned row ID remain the same as generated autoincrement id?
– UnknownJoe
Jan 28 '14 at 9:07
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Number of rows affected by an UPDATE in PL/SQL
...s with the number of line affected by the last DML operation:
say we have table CLIENT
create table client (
val_cli integer
,status varchar2(10)
)
/
We would test it this way:
begin
dbms_output.put_line('Value when entering the block:'||sql%rowcount);
insert into client
se...
ORA-00979 not a group by expression
...ave a database like this:
FOO BAR
0 A
0 B
and you run SELECT * FROM table GROUP BY foo. This means the database must return a single row as result with the first column 0 to fulfill the GROUP BY but there are now two values of bar to chose from. Which result would you expect - A or B? Or shou...
How do I check if a column is empty or null in MySQL?
I have a column in a table which might contain null or empty values. How do I check if a column is empty or null in the rows present in a table?
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When/Why to use Cascading in SQL Server?
...eal key rather than a surrogate key (identity/autoincrement column) across tables.
The canonical example for Cascade Update is when you have a mutable foreign key, like a username that can be changed.
You should not use Cascade Update with keys that are Identity/autoincrement columns.
Cascade Updat...
How do I horizontally center a span element inside a div
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another option would be to give the span display:table; and center it via margin:0 auto;
span {
display:table;
margin:0 auto;
}
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What does the “@” symbol do in Powershell?
...l V2, @ is also the Splat operator.
PS> # First use it to create a hashtable of parameters:
PS> $params = @{path = "c:\temp"; Recurse= $true}
PS> # Then use it to SPLAT the parameters - which is to say to expand a hash table
PS> # into a set of command line parameters.
PS> dir @para...
How to check if a database exists in SQL Server?
...ing the INFORMATION_SCHEMA views rather than directly accessing the system tables.
– mwigdahl
Mar 24 '09 at 20:09
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