大约有 37,000 项符合查询结果（耗时：0.0228秒） [XML]
Count number of rows within each group
...
An old question without a data.table solution. So here goes...
Using .N
library(data.table)
DT <- data.table(df)
DT[, .N, by = list(year, month)]
share
|
...
Rails: Using greater than/less than with a where statement
...non discrete values or columns where you can't use the + 1 strategy.
Arel table
If you want to get fancy you can make use of the Arel::Table.
User.where(User.arel_table[:id].gt(200))
will generate the SQL
"SELECT `users`.* FROM `users` WHERE (`users`.`id` > 200)"
The specifics are as foll...
Finding duplicate values in a SQL table
...till requires all non-aggregated columns in the GROUP BY.
MySQL is unpredictable and you need sql_mode=only_full_group_by:
GROUP BY lname ORDER BY showing wrong results;
Which is the least expensive aggregate function in the absence of ANY() (see comments in accepted answer).
Oracle isn't mainstr...
How do I handle too long index names in a Ruby on Rails ActiveRecord migration?
...an unique index that gets created from the foreign keys of four associated tables:
9 Answers
...
TSQL Pivot without aggregate function
I have a table like this...
9 Answers
9
...
Get the last inserted row ID (with SQL statement) [duplicate]
I want to get the new created ID when you insert a new record in table.
3 Answers
3
...
Is there a way to get a collection of all the Models in your Rails app?
...
ActiveRecord::Base.connection.tables.map do |model|
model.capitalize.singularize.camelize
end
will return
["Article", "MenuItem", "Post", "ZebraStripePerson"]
Additional information If you want to call a method on the object name without model:stri...
Can I create a named default constraint in an add column statement in SQL Server?
In SQL Server, I have a new column on a table:
4 Answers
4
...
What is the reason not to use select *?
...nt, the SQL execution engine will error if that column is removed from the table and the query is executed.
You can more easily scan code where that column is being used.
You should always write queries to bring back the least amount of information.
As others mention if you use ordinal column access...
Why do I get a “Null value was assigned to a property of primitive type setter of” error message whe
...he solution is replace primitive type with primitive wrapper class in your tableName.java file.
Such as:
@Column(nullable=true, name="client_os_id")
private Integer client_os_id;
public int getClient_os_id() {
return client_os_id;
}
public void setClient_os_id(int clientOsId) {
client_os_...