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Repeat each row of data.frame the number of times specified in a column
...ndRows(df, "freq")
Simple syntax, very fast, works on data.frame or data.table.
Result:
var1 var2
1 a d
2 b e
2.1 b e
3 c f
3.1 c f
3.2 c f
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Is there any significant difference between using if/else and switch-case in C#?
... in debug or compatibility mode. In release, it will be compiled into jump table (through MSIL 'switch' statement)- which is O(1).
C# (unlike many other languages) also allows to switch on string constants - and this works a bit differently. It's obviously not practical to build jump tables for str...
Want to find records with no associated records in Rails
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Just imagine you have 10000000 records in friends table. What about performance in that case?
– goodniceweb
Dec 20 '16 at 14:51
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Retrieving the last record in each group - MySQL
There is a table messages that contains data as shown below:
27 Answers
27
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Rails: Adding an index after adding column
Suppose I created a table table in a Rails app. Some time later, I add a column running:
5 Answers
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How can I add a column that doesn't allow nulls in a Postgresql database?
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You have to set a default value.
ALTER TABLE mytable ADD COLUMN mycolumn character varying(50) NOT NULL DEFAULT 'foo';
... some work (set real values as you want)...
ALTER TABLE mytable ALTER COLUMN mycolumn DROP DEFAULT;
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Possible to perform cross-database queries with PostgreSQL?
...them.
postgres_fdw
Use postgres_fdw (foreign data wrapper) to connect to tables in any Postgres database - local or remote.
Note that there are foreign data wrappers for other popular data sources. At this time, only postgres_fdw and file_fdw are part of the official Postgres distribution.
For P...
Rename column SQL Server 2008
I am using SQL Server 2008 and Navicat. I need to rename a column in a table using SQL.
11 Answers
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How to do an update + join in PostgreSQL?
...ATE syntax is:
[ WITH [ RECURSIVE ] with_query [, ...] ]
UPDATE [ ONLY ] table [ [ AS ] alias ]
SET { column = { expression | DEFAULT } |
( column [, ...] ) = ( { expression | DEFAULT } [, ...] ) } [, ...]
[ FROM from_list ]
[ WHERE condition | WHERE CURRENT OF cursor_name ]
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What's the difference between MyISAM and InnoDB? [duplicate]
...re, but most of the time it is asked in relation to a specific database or table. I cannot find an answer on this site that describes the two engines and their differences without respect to someones specific database.
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