大约有 38,000 项符合查询结果（耗时：0.0239秒） [XML]
Facebook database design?
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Keep a friend table that holds the UserID and then the UserID of the friend (we will call it FriendID). Both columns would be foreign keys back to the Users table.
Somewhat useful example:
Table Name: User
Columns:
UserID PK
Emai...
LINQ to SQL: Multiple joins ON multiple Columns. Is this possible?
...olumns in Linq to SQL is a little different.
var query =
from t1 in myTABLE1List // List<TABLE_1>
join t2 in myTABLE1List
on new { t1.ColumnA, t1.ColumnB } equals new { t2.ColumnA, t2.ColumnB }
...
You have to take advantage of anonymous types and compose a type for the mu...
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...
SQL Server - Return value after INSERT
... get a the key-value back after an INSERT-statement.
Example:
I've got a table with the attributes name and id. id is a generated value.
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MySQL offset infinite rows
I would like to construct a query that displays all the results in a table, but is offset by 5 from the start of the table. As far as I can tell, MySQL's LIMIT requires a limit as well as an offset. Is there any way to do this?
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How can I rename a database column in a Ruby on Rails migration?
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rename_column :table, :old_column, :new_column
You'll probably want to create a separate migration to do this. (Rename FixColumnName as you will.):
script/generate migration FixColumnName
# creates db/migrate/xxxxxxxxxx_fix_column_name....
SQL/mysql - Select distinct/UNIQUE but return all columns?
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You're looking for a group by:
select *
from table
group by field1
Which can occasionally be written with a distinct on statement:
select distinct on field1 *
from table
On most platforms, however, neither of the above will work because the behavior on the other co...
Can someone explain mappedBy in JPA and Hibernate?
...irline that it is responsible for maintaining the values over in the other table. It is possible to tell an Entity it "owns" a column in a different table and is responsible for updating it. It's not something you usually want to do and can cause problems with the order SQL statements are executed...
generate days from date range
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This solution uses no loops, procedures, or temp tables. The subquery generates dates for the last 10,000 days, and could be extended to go as far back or forward as you wish.
select a.Date
from (
select curdate() - INTERVAL (a.a + (10 * b.a) + (100 * c.a) + (1000 * d...
How to tell when UITableView has completed ReloadData?
I am trying to scroll to the bottom of a UITableView after it is done performing [self.tableView reloadData]
19 Answers
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