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When and why are database joins expensive?
...s back to mitigating the size of the working set. Joins involving properly selected keys with correctly set up indexes are cheap, not expensive, because they allow significant pruning of the result before the rows are materialised.
Materialising the result involves bulk disk reads which are the mo...
来自微软的一手内幕：Windows 10是怎么出炉的 - 创意 - 清泛网 - 专注C/C++及内核技术
...，将代替每隔几年的大升级。微软未必真知道，此举终将如何收场，但Windows 10的出现，注定成为一个里程碑。
这家公司将再不能承受Windows 8式失误造成的恶果
这里程碑不同以往。即便Windows 10已经发布，也不算万事大...
How to find all tables that have foreign keys that reference particular table.column and have values
...
Here you go:
USE information_schema;
SELECT *
FROM
KEY_COLUMN_USAGE
WHERE
REFERENCED_TABLE_NAME = 'X'
AND REFERENCED_COLUMN_NAME = 'X_id';
If you have multiple databases with similar tables/column names you may also wish to limit your query to a particul...
Get current AUTO_INCREMENT value for any table
...'TableName' ;
You can get exactly this information by using this query:
SELECT `AUTO_INCREMENT`
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_SCHEMA = 'DatabaseName'
AND TABLE_NAME = 'TableName';
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How to find duplicates in 2 columns not 1
..._title for each row.
As far as finding the existing duplicates try this:
select stone_id,
upcharge_title,
count(*)
from your_table
group by stone_id,
upcharge_title
having count(*) > 1
...
Unknown Column In Where Clause
...om right to left. So the where clause is parsed and evaluate prior to the select clause. Because of this the aliasing of u_name to user_name has not yet occurred.
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...
SQL UPDATE SET one column to be equal to a value in a related table referenced by a different column
...result set to update is before running the update (same query, just with a select):
select *
from QuestionTrackings q
inner join QuestionAnswers a
on q.AnswerID = a.AnswerID
where q.QuestionID is null -- and other conditions you might want
Particularly whether each answer id has definitely only 1...
MySQL Like multiple values
...
To get the regexp value from a column: (select group_concat(myColumn separator '|') from..)
– daVe
Nov 28 '15 at 1:06
|...
Two single-column indexes vs one two-column index in MySQL?
... the statement I provided.
Additionally, MySQL can only use one index per SELECT so a covering index would be the best means of optimizing your queries.
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Find and Replace text in the entire table using a MySQL query
...his_text";
$replace = "replace_with_this_text";
$loop = mysql_query("
SELECT
concat('UPDATE ',table_schema,'.',table_name, ' SET ',column_name, '=replace(',column_name,', ''{$find}'', ''{$replace}'');') AS s
FROM
information_schema.columns
WHERE
table_schema = '{...