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SQL Server indexes - ascending or descending, what difference does it make?
...rily matters when used with composite indexes:
CREATE INDEX ix_index ON mytable (col1, col2 DESC);
can be used for either:
SELECT *
FROM mytable
ORDER BY
col1, col2 DESC
or:
SELECT *
FROM mytable
ORDER BY
col1 DESC, col2
, but not for:
SELECT *
FROM mytable
ORDE...
Change a column type from Date to DateTime during ROR migration
...
First in your terminal:
rails g migration change_date_format_in_my_table
Then in your migration file:
For Rails >= 3.2:
class ChangeDateFormatInMyTable < ActiveRecord::Migration
def up
change_column :my_table, :my_column, :datetime
end
def down
change_column :my_tabl...
How should I store GUID in MySQL tables?
...the intention of representing said value with a value mapped from a lookup table (in most cases now, UTF8). A BINARY field expects the same kind of value without any intention of representing said data from a lookup table. I used CHAR(16) back in the 4.x days because back then MySQL wasn't as good a...
App Inventor 2 字典 vs 列表：同样存数据，性能差了10倍？ - App应用开发 ...
...><hr><br><b>逐一 PK：字典 vs 列表核心能力对比</b><br><br><br><table border="1" cellpadding="4" cellspacing="0" style="border-collapse:collapse;"><tbody><tr><th>对比维度</th><th>字典 (Dictionary)</th><th>列表 (List)</th></tr><tr><td><b>数据组织方式</b></td><td>键值映射<...
Set default value of an integer column SQLite
...
Use the SQLite keyword default
db.execSQL("CREATE TABLE " + DATABASE_TABLE + " ("
+ KEY_ROWID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
+ KEY_NAME + " TEXT NOT NULL, "
+ KEY_WORKED + " INTEGER, "
+ KEY_NOTE + " INTEGER DEFAULT 0);");
This link is useful: ht...
Why is early return slower than else?
...ashing works:
__builtins__ hashes to -1196389688 which reduced modulo the table size (32) means it is stored in the #8 slot of the table.
without_else hashes to 505688136 which reduced modulo 32 is 8 so there's a collision. To resolve this Python calculates:
Starting with:
j = hash % 32
perturb ...
Why is LINQ JOIN so much faster than linking with WHERE?
...nce they work directly in memory (Linq to DataSet)
The query with multiple tables and a Where condition actually performs a cartesian product of all the tables, then filters the rows that satisfy the condition. This means the Where condition is evaluated for each combination of rows (n1 * n2 * n3 * ...
UITableview: How to Disable Selection for Some Rows but Not Others
I am displaying in a group tableview contents parsed from XML. I want to disable the click event on it (I should not be able to click it at all) The table contains two groups. I want to disable selection for the first group only but not the second group. Clicking the first row of second group nav...
Creating and Update Laravel Eloquent
...built in L4 functionality for doing this in some way such as:
$row = DB::table('table')->where('id', '=', $id)->first();
// Fancy field => data assignments here
$row->save();
I did create this method a few weeks back...
// Within a Model extends Eloquent
public static function creat...
Remove duplicated rows
...
The data.table package also has unique and duplicated methods of it's own with some additional features.
Both the unique.data.table and the duplicated.data.table methods have an additional by argument which allows you to pass a chara...