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常用Sql - 爬虫/数据库 - 清泛IT社区，为创新赋能！
本帖最后由 zqp2013 于 2015-3-17 22:06 编辑
mysql：drop table if exists tablename;
不能写成
drop table tablename if exists tablename;
mysql：建立索引Sql
CREATE TABLE tablename (
`ID` &nbs...
Switch on Enum in Java [duplicate]
...r speed, the switch statement approach might be faster because it's just a table lookup in the bytecode. With your approach, you have to push a new function onto the stack. It's a micro-optimization, but if speed is super important it's something to keep in mind.
– DIMMSum
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ASP.NET MVC 5 - Identity. How to get current ApplicationUser
... having an extra context for starters, but going forward the user database tables change (3 times in the past 2 years) but the API is consistent. For example the users table is now called AspNetUsers in Identity Framework, and the names of several primary key fields kept changing, so the code in sev...
Use variable with TOP in select statement in SQL Server without making it dynamic [duplicate]
...'s possible to use a variable in the top clause.
select top (@top) * from tablename
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SQLite string contains other string query
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Using LIKE:
SELECT *
FROM TABLE
WHERE column LIKE '%cats%' --case-insensitive
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postgresql COUNT(DISTINCT …) very slow
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You can use this:
SELECT COUNT(*) FROM (SELECT DISTINCT column_name FROM table_name) AS temp;
This is much faster than:
COUNT(DISTINCT column_name)
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What happens when a computer program runs?
...me systems)
+---------+
| text | program code, this is the actual executable code that is running.
+---------+
This is the general process address space on many common virtual-memory systems. The "hole" is the size of your total memory, minus the space taken up by all the other areas; this gi...
How to best display in Terminal a MySQL SELECT returning too many fields?
...erminate the query with \G in place of ;. For example:
SELECT * FROM sometable\G
This query displays the rows vertically, like this:
*************************** 1. row ***************************
Host: localhost
Db: mydatabase1
User: myuser1
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How to convert local time string to UTC?
... platform. You could use tzlocal.get_localzone() to provide tzdata in a portable way. (2) fromtimestamp(t, tz) may fail for non-pytz timezones. (3) datetime(*struct_time[:6]) you are missing *. (4) timegm(), utctimetuple(), struct_time -based solutions drop fractions of a second. You could use an e...
MySql export schema without data
...nsider using the --single-transaction option if you don't want or can't do table locks.
– Jim
Jan 22 '13 at 21:55
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