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理解Python的 with 语句 - 大数据 & AI - 清泛网 - 专注C/C++及内核技术
...码：
with open("/tmp/foo.txt") as file:
data = file.read()
with如何工作？
while this might look like magic, the way Python handles with is more clever than magic. The basic idea is that the statement after with has to evaluate an object that responds to an __enter__() as well as an ...
Updating MySQL primary key
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mysql Foreign key constraint is incorrectly formed error
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mysql error texts doesn't help so much, in my case, the column had "not null" constraint, so the "on delete set null" was not allowed
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Insert into a MySQL table or update if exists
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Check out REPLACE
http://dev.mysql.com/doc/refman/5.0/en/replace.html
REPLACE into table (id, name, age) values(1, "A", 19)
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What is the “N+1 selects problem” in ORM (Object-Relational Mapping)?
...cally, a lot of dbms have been quite poor when it comes to handling joins (MySQL being a particularly noteworthy example). So n+1 has, often, been notably faster than a join. And then there are ways to improve on n+1 but still without needing a join, which is what the original problem relates to.
H...
How to change the foreign key referential action? (behavior)
...re
Example:
I have a UserDetails table refers to Users table:
mysql> SHOW CREATE TABLE UserDetails;
:
:
`User_id` int(11) DEFAULT NULL,
PRIMARY KEY (`Detail_id`),
KEY `FK_User_id` (`User_id`),
CONSTRAINT `FK_User_id` FOREIGN KEY (`User_id`) REFERENCES `Users` (`User_id`)
:
:
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mysql - how many columns is too many?
...tables with many rows, modifying the indexes can take a very long time, as MySQL needs to rebuild all of the indexes in the table. Having the indexes split over several table could make that faster.
Depending on your queries and column types, MySQL could be writing temporary tables (used in more co...
How can I do an UPDATE statement with JOIN in SQL Server?
...Here are some ways to do it in ANSI/ISO (aka should work on any SQL DBMS), MySQL, SQL Server, and Oracle. Be advised that my suggested ANSI/ISO method will typically be much slower than the other two methods, but if you're using a SQL DBMS other than MySQL, SQL Server, or Oracle, then it may be the ...
MySQL IF NOT NULL, then display 1, else display 0
... NOT NULL) AS addressexists
This works because TRUE is displayed as 1 in MySQL and FALSE as 0.
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MySQL - length() vs char_length()
...ore accurately UTF-16LE) is what Windows misleadingly calls “Unicode”. MySQL doesn't support UTF-16; instead the usual approach for putting Unicode strings in it is to use UTF-8.
– bobince
Nov 14 '09 at 14:20
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