大约有 40,000 项符合查询结果（耗时：0.0311秒） [XML]
When should I use a composite index?
...ly benefit queries that query on both dimensions or the leftmost dimension by itself, but not the rightmost dimension by itself. If you're always querying two dimensions, a composite index is the way to go, doesn't really matter which is first (most probably).
...
How to delete a certain row from mysql table with same column values?
... solves your current issue, you should definitely work on the bigger issue by defining primary keys.
share
|
improve this answer
|
follow
|
...
Overriding id on create in ActiveRecord
...at for picklists and small tables that change infrequently and only change by developers. So when going from ActiveHash to ActiveRecord, it's easiest to just keep all of the foreign key references the same.
share
|...
Possible to do a MySQL foreign key to one of two possible tables?
...lways references one target table.
Polymorphic Associations are supported by frameworks such as Rails and Hibernate. But they explicitly say that you must disable SQL constraints to use this feature. Instead, the application or framework must do equivalent work to ensure that the reference is sat...
SQL JOIN and different types of JOINs
...hey have taken the idea for the pictures. See A visualization of SQL joins by Jeff Atwood (yes, the one who co-authored SO) and the linked article by Ligaya Turmelle where Jeff got the idea and explanded it.
– ypercubeᵀᴹ
Oct 12 '14 at 11:29
...
Bundle ID Suffix? What is it?
...n the Developer Portal:
The App ID string contains two parts separated by a period (.) — an App ID Prefix (your Team ID by default, e.g. ABCDE12345), and an App ID Suffix (a Bundle ID search string, e.g. com.mycompany.appname). [emphasis added]
So in this case the suffix is the full string c...
How to include “zero” / “0” results in COUNT aggregate?
...
LEFT JOIN appointment ON person.person_id = appointment.person_id
GROUP BY person.person_id;
The reason why this is working, is that the outer (left) join will return NULL for those persons that do not have an appointment. The aggregate function count() will not count NULL values and thus you'l...
unique object identifier in javascript
...for..in loop. This is a well documented practice and one that is enforced by jslint
– Justin Johnson
Jan 4 '10 at 23:01
3
...
How to get multiple counts with one SQL query?
...evel = 'personal' then 1 else 0 end) AS PersonalCount
FROM yourtable
GROUP BY distributor_id
share
|
improve this answer
|
follow
|
...
How to reset sequence in postgres and fill id column with new data?
...s no need to specify the WITH 1 anymore. The start value that was recorded by CREATE SEQUENCE or last set by ALTER SEQUENCE START WITH will be used (most probably this will be 1).
Reset the sequence:
ALTER SEQUENCE seq RESTART;
Then update the table's ID column:
UPDATE foo SET id = DEFAULT;
S...