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Relative frequencies / proportions with dplyr
...our depending on the version of dplyr:
dplyr 0.7.1: returns an ungrouped table: you need to group again by am
dplyr < 0.7.1: returns a grouped table, so no need to group again, although you might want to ungroup() for later manipulations
dplyr 0.7.1
mtcars %>%
count(am, gear) %>%
g...
How to check if a Constraint exists in Sql server?
...QUE, PRIMARY KEY, and/or FOREIGN KEY
SELECT *
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
WHERE CONSTRAINT_NAME='XYZ'
--Returns one row for each FOREIGN KEY constrain
SELECT *
FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS
WHERE CONSTRAINT_NAME='XYZ'
--Returns one row for eac...
What is the advantage to using bloom filters?
...esenting sets, such as
self-balancing binary search trees,
tries, hash tables, or simple arrays
or linked lists of the entries. Most
of these require storing at least the
data items themselves, which can
require anywhere from a small number
of bits, for small integers, to an
arbitrar...
How to alter a column's data type in a PostgreSQL table?
...entation here: http://www.postgresql.org/docs/current/interactive/sql-altertable.html
ALTER TABLE tbl_name ALTER COLUMN col_name TYPE varchar (11);
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Pass column name in data.table using variable [duplicate]
In following example, I am creating a data table having column name ‘x’ and ‘v’
1 Answer
...
Oracle的列操作(增加列,修改列,删除列)，包括操作多列 - 数据库(内核) - 清...
...(增加列,修改列,删除列)，包括操作多列增加一列： alter table emp4 add test varchar2(10);修改一列： alter table emp4 modify test varchar2(20);删除一列：alter 增加一列：
alter table emp4 add test varchar2(10);
修改一列：
alter table emp4 modify test va...
Combining “LIKE” and “IN” for SQL Server [duplicate]
...y, the IN statement creates a series of OR statements... so
SELECT * FROM table WHERE column IN (1, 2, 3)
Is effectively
SELECT * FROM table WHERE column = 1 OR column = 2 OR column = 3
And sadly, that is the route you'll have to take with your LIKE statements
SELECT * FROM table
WHERE column...
How to wrap text using CSS? [duplicate]
...
Try doing this. Works for IE8, FF3.6, Chrome
<body>
<table>
<tr>
<td>
<div style="word-wrap: break-word; width: 100px">gdfggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggggg...
How to map a composite key with JPA and Hibernate?
...// getters, setters
}
The IdClass annotation maps multiple fields to the table PK.
With EmbeddedId
The class for the composite primary key could look like (could be a static inner class):
@Embeddable
public class TimePK implements Serializable {
protected Integer levelStation;
protecte...
Why is Dictionary preferred over Hashtable in C#?
In most programming languages, dictionaries are preferred over hashtables.
What are the reasons behind that?
19 Answers
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