大约有 3,551 项符合查询结果（耗时：0.0171秒） [XML]
How to join (merge) data frames (inner, outer, left, right)
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@MattParker I have been using sqldf package for a whole host of complex queries against dataframes, really needed it to do a self-cross join (ie data.frame cross-joining itself) I wonder how it compares from a performance perspective....???
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update columns values with column of another table based on condition [duplicate]
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it's not working in mysql
– cikatomo
Sep 30 '15 at 19:30
22
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How to alter a column's data type in a PostgreSQL table?
Entering the following command into a PostgreSQL interactive terminal results in an error:
1 Answer
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Can I change a column from NOT NULL to NULL without dropping it?
... I tried with alter column and it didn't work with the latest mysql server. I tried with modify column and it worked. Strange...
– Kostas Andrianos
Aug 19 '17 at 15:29
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Set value to NULL in MySQL
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You're probably quoting 'NULL'. NULL is a reserved word in MySQL, and can be inserted/updated without quotes:
INSERT INTO user (name, something_optional) VALUES ("Joe", NULL);
UPDATE user SET something_optional = NULL;
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New transaction is not allowed because there are other threads running in the session LINQ To Entity
...oblem is not specific to Entity Framework. It is related to the underlying SQL Server SqlConnection object. Evidently, in this scenario, Entity framework sees fit to start a transaction for each save. This is a terrible idea, and an argument for not using Entity Framework. My guess is that the code ...
How to create user for a db in postgresql? [closed]
I have installed PostgreSQL 8.4 on my CentOS server and connected to root user from shell and accessing the PostgreSQL shell.
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千万别惹程序员 - 轻松一刻 - 清泛网 - 专注C/C++及内核技术
...些精细的调优工作，明显不行。这和Ruby很像。
PHP没有MySQL，明显是被幽默了一把。不过最近对PHP的批评越来越多，不过，facebook的PHP的引擎HiPo已经很牛B了。
Perl是一本日本武士刀，是忍者玩的语言。
VB，就是一个玩具。你见...
半个汉字的校验与处理(C++) - C/C++ - 清泛网 - 专注C/C++及内核技术
...串的后续处理，会带来一定的麻烦，比如在xml文档、或者SQL语句中，如果存在半个汉字，就会导致xml解析失败，或者sql执行错误的异常。为了解决这个问题，有必要找到并删除半个汉字，或者存放的时候，即便截断，也不产生新...
Oracle 分组后取每组第一条数据 - 数据库(内核) - 清泛网 - 专注C/C++及内核技术
...组后取每组第一条数据Oracle 分组后取每组第一条数据的SQL如下，亲测有效：SELECT * FROM (SELECT ROW_NUMBER() OVER(PARTITION BY T field1, T Oracle 分组后取每组第一条数据的SQL如下，亲测有效：
SELECT *
FROM (SELECT ROW_NUMBER() OVER(PARTITION ...