大约有 47,000 项符合查询结果（耗时：0.0433秒） [XML]
What does SQL clause “GROUP BY 1” mean?
...
SELECT account_id, open_emp_id
^^^^ ^^^^
1 2
FROM account
GROUP BY 1;
In above query GROUP BY 1 refers to the first column in select statement which is
account_id.
You also can speci...
Show constraints on tables command
...
Simply query the INFORMATION_SCHEMA:
USE INFORMATION_SCHEMA;
SELECT TABLE_NAME,
COLUMN_NAME,
CONSTRAINT_NAME,
REFERENCED_TABLE_NAME,
REFERENCED_COLUMN_NAME
FROM KEY_COLUMN_USAGE
WHERE TABLE_SCHEMA = "<your_database_name>"
AND TABLE_NAME = "<y...
How do I extract a sub-hash from a hash?
...e same:
h1 = {:a => :A, :b => :B, :c => :C, :d => :D}
h2 = h1.select {|key, value| [:b, :d, :e, :f].include?(key) } # => {:b=>:B, :d=>:D}
h1 = Hash[h1.to_a - h2.to_a] # => {:a=>:A, :c=>:C}
And if you want to patch that into the Hash class:
class Hash
def extract...
MySQL CONCAT returns NULL if any field contain NULL
...
convert the NULL values with empty string by wrapping it in COALESCE
SELECT CONCAT(COALESCE(`affiliate_name`,''),'-',COALESCE(`model`,''),'-',COALESCE(`ip`,''),'-',COALESCE(`os_type`,''),'-',COALESCE(`os_version`,'')) AS device_name
FROM devices
...
SQL Server Installation - What is the Installation Media Folder?
... Server 2019 Developer version. Was lost with dialog prompted from step 3 "Select installation media". This Andy Leonard blog helped me: 1. Downloaded installation package from SQL Server downloads. 2. Launch downloaded wizard > Select installation type > Download media 3. Right-click on the d...
Multiline strings in VB.NET
...
I used this variant:
Dim query As String = <![CDATA[
SELECT
a.QuestionID
FROM
CR_Answers a
INNER JOIN
CR_Class c ON c.ClassID = a.ClassID
INNER JOIN
CR_Questions q ON q.QuestionID = a.QuestionID
...
CSS selector for first element with class
I have a bunch of elements with a class name red , but I can't seem to select the first element with the class="red" using the following CSS rule:
...
Angular ng-repeat Error “Duplicates in a repeater are not allowed.”
...cause I mistakenly set the ng-model the same as the ng-repeat array:
<select ng-model="list_views">
<option ng-selected="{{view == config.list_view}}"
ng-repeat="view in list_views"
value="{{view}}">
{{view}}
</option>
</select>
Inste...
How to implement classic sorting algorithms in modern C++?
...ed and hybrid amalgamation of more elementary sorting algorithms , such as selection sort, insertion sort, quick sort, merge sort, or heap sort.
...
How do I find which transaction is causing a “Waiting for table metadata lock” state?
...about all the locks transactions are waiting for:
USE INFORMATION_SCHEMA;
SELECT * FROM INNODB_LOCK_WAITS;
A list of blocking transactions:
SELECT *
FROM INNODB_LOCKS
WHERE LOCK_TRX_ID IN (SELECT BLOCKING_TRX_ID FROM INNODB_LOCK_WAITS);
OR
SELECT INNODB_LOCKS.*
FROM INNODB_LOCKS
JOIN INNOD...