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What is causing this ActiveRecord::ReadOnlyRecord error?
... you use this option, saving the same
record will now fail. Use find_by_sql
to work around.
Using find_by_sql is not really an alternative as it returns raw row/column data, not ActiveRecords. You have two options:
Force the instance variable @readonly to false in the record (hack)
Use :in...
Postgres: INSERT if does not exist already
...
@TusharJain prior to PostgreSQL 9.5 you can do an "old fashioned" UPSERT (with CTE) but you may experience problems with race conditions and it will not be performant as 9.5 style. There is a good detail about upsert on this blog (in the updated area at...
Foreign key constraint may cause cycles or multiple cascade paths?
...
SQL Server does simple counting of cascade paths and, rather than trying to work out whether any cycles actually exist, it assumes the worst and refuses to create the referential actions (CASCADE): you can and should still cr...
MySQL ON vs USING?
In a MySQL JOIN , what is the difference between ON and USING() ? As far as I can tell, USING() is just more convenient syntax, whereas ON allows a little more flexibility when the column names are not identical. However, that difference is so minor, you'd think they'd just do away with USI...
MySQL: selecting rows where a column is null
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SQL NULL's special, and you have to do WHERE field IS NULL, as NULL cannot be equal to anything,
including itself (ie: NULL = NULL is always false).
See Rule 3 https://en.wikipedia.org/wiki/Codd%27s_12_rules
...
Which are more performant, CTE or temporary tables?
...y to other constructs that can also be used as inline table expressions in SQL Server. Derived tables, Views, and inline table valued functions. Note that whilst BOL says that a CTE "can be thought of as temporary result set" this is a purely logical description. More often than not it is not materl...
程序员之网络安全系列（二）：如何安全保存用户密码及哈希算法 - 更多技术 ...
...段数据唯一且极其紧凑的数值表示形式. 典型的哈希算法包括 MD2、MD4、MD5 和 SHA-1
Hash算法是给消息生成摘要，那么什么是摘要呢？
举个例子： > 比如你给你女朋友写了一封邮件，确保没被人改过，你可以生成这样一份摘要 “...
How to escape apostrophe (') in MySql?
The MySQL documentation says that it should be \' . However, both scite and mysql shows that '' works. I saw that and it works. What should I do?
...
Copy a table from one database to another in Postgres
...it directly to the target database:
pg_dump -t table_to_copy source_db | psql target_db
Note: If the other database already has the table set up, you should use the -a flag to import data only, else you may see weird errors like "Out of memory":
pg_dump -a -t my_table my_db | psql target_db
...
SQL Server indexes - ascending or descending, what difference does it make?
When you create an index on a column or number of columns in MS SQL Server (I'm using version 2005), you can specify that the index on each column be either ascending or descending. I'm having a hard time understanding why this choice is even here. Using binary sort techniques, wouldn't a lookup be ...