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When and why are database joins expensive?
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MySQL, Oracle, SQL Server, Sybase, postgreSQL,etc. care not the order of joins. I've worked with DB2 and it also, to my knowledge, doesn't care what order you put them in. This is not helpful advice in the general case
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When should I use a composite index?
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@felwithe MySQL can only use one index per each of the tables in a query (Exemptions are there. eg, Merging of indexes). Which ideally means that a table in a query, must use a single index for all where-clause, table join, group-by an...
How do I check if a column is empty or null in MySQL?
...OR some_col = ' ' (one space inserted in the string) then it works on both MySQL and Oracle, see answer of "onedaywhen". some_col = '' doesn't work on Oracle as empty strings mean NULL there.
– Johanna
May 18 '15 at 14:07
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MySQL “between” clause not inclusive?
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From the MySQL-manual:
This is equivalent to the expression
(min <= expr AND expr <= max)
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What is the difference between tree depth and height?
... number of nodes and in other number of edges on the shortest path between root and concrete node.
Which is which?
7 Answ...
What can you use Python generator functions for?
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Real World Example
Let's say you have 100 million domains in your MySQL table, and you would like to update Alexa rank for each domain.
First thing you need is to select your domain names from the database.
Let's say your table name is domains and column name is domain.
If you use SELECT...
How do I change the data type for a column in MySQL?
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http://dev.mysql.com/doc/refman/5.1/en/alter-table.html
ALTER TABLE tablename MODIFY columnname INTEGER;
This will change the datatype of given column
Depending on how many columns you wish to modify it might be best to generate a scri...
App Inventor 2 DynamicComponents 拓展：动态创建AI2组件对象 · App Inventor 2 中文网
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通过 JSON 字符串创建组件。请参阅Wiki，了解如何创建自己的模板。
从屏幕和组件列表中移除指定 ID 的组件。由于该组件已被删除，因此你可以再次使用该 ID...
PDOException “could not find driver”
I have just installed Debian Lenny with Apache, MySQL, and PHP and I am receiving a PDOException could not find driver .
3...
Creating a simple XML file using python
...nt using the in-stdlib cElementTree:
import xml.etree.cElementTree as ET
root = ET.Element("root")
doc = ET.SubElement(root, "doc")
ET.SubElement(doc, "field1", name="blah").text = "some value1"
ET.SubElement(doc, "field2", name="asdfasd").text = "some vlaue2"
tree = ET.ElementTree(root)
tree.wr...