relational

How does `std::less` work?

由 风格不统一 提交于 2019-12-09 10:10:16
问题 Pointer relational operators do not define a total order (§ 5.9 of the C++11 standard): If two pointers p and q of the same type point to different objects that are not members of the same object or elements of the same array or to different functions, or if only one of them is null, the results of p<q , p>q , p<=q , and p>=q are unspecified. std::less documentation says: The partial specialization of std::less for any pointer type yields a total order, even if the built-in operator< does not

How can I access the last inserted row ID within a SQL script?

由 耗尽温柔 提交于 2019-12-08 15:51:35
问题 I'm using SQLite, and I have a table for properties, and a table for sub-properties. Each sub-property points to its parent using the fkPropertyId column. Right now, to create the initial database, I've got a script that looks something like this: INSERT INTO property VALUES(1,.....); INSERT INTO property VALUES(2,.....); INSERT INTO property VALUES(3,.....); INSERT INTO subproperty VALUES(1,.....,3); INSERT INTO subproperty VALUES(2,.....,3); INSERT INTO subproperty VALUES(3,.....,3); INSERT

How to store objects in a relational database? [closed]

由 风格不统一 提交于 2019-12-08 12:22:45
问题 Closed . This question is opinion-based. It is not currently accepting answers. Want to improve this question? Update the question so it can be answered with facts and citations by editing this post. Closed 6 years ago . Someone asked a similar question before, but it wasn't answered. How to store value objects in a relational database? Here is an example situation where my question comes into play. Let's say there is a 'users' table, and each user needs to have their location stored. Their

In SQL, how do you get the top N rows ordered by a certain column?

由 半腔热情 提交于 2019-12-07 18:28:26
问题 I want to select the top N rows of a table, ranked by how high the number in one of their columns is. I already have: SELECT * FROM movie ORDER BY worldwide_gross DESC; How can I get the first twenty? If it makes any difference I'm using MySQL. Cheers! 回答1: Definition: Limit is used to limit your MySQL query results to those that fall within a specified range. You can use it to show the first X number of results, or to show a range from X - Y results. It is phrased as Limit X, Y and included

Retrieve maximal / minimal record

由 夙愿已清 提交于 2019-12-07 08:39:09
问题 A rather complicated SQL query I was working on got me thinking about a limitation of (ANSI) SQL: Is there a way to retrieve a record that is maximal or minimal with respect to an arbitrary ordering? In other words: Given a query like this: SELECT * FROM mytable WHERE <various conditions> ORDER BY <order clause> is it possible to write a query that returns only the first row (possibly by transforming the order clause into something else)? I know you can do this using LIMIT (MySQL) / ROWNUM

Terribly slow SQL query with COUNT and GROUP BY on two columns

由 你。 提交于 2019-12-06 05:16:37
问题 I'm archiving this web forum, which normally gets purged about once a week. So I'm screen scraping it, and storing it into my database (PostgreSQL). I also do a little analysis on the data, with some graphs for users to enjoy, like what time of day is the forum most active, and so forth. So I have a posts table, like so: Column | Type ------------+------------------------------ id | integer body | text created_at | timestamp without time zone topic_id | integer user_name | text user_id |

Relational DB in-memory?

由 二次信任 提交于 2019-12-04 22:23:34
问题 I have a simpleton question on Redis. If the key to it's performance is that it's in-memory, whey can't that be done on a regular SQL db? 回答1: Any DBMS can be run "in memory". Consider the use of a ramdisk. However, most DBMSs (those with SQL) are not designed to run entirely in memory and put alot of effort to minimize disk IO and paging: a DBMS works very hard to keep the "relevant data" hot (in memory and in cache) -- IO is slow, slow slow. This is because database data is often [and has

How do you store a trie in a relational database?

由 若如初见. 提交于 2019-12-04 16:24:12
问题 I have a prefix trie. What is the recommended schema for representing this structure in a relational database? I need substring matching to remain efficient. 回答1: How about the Materialized Path design? CREATE TABLE trie ( path VARCHAR(<maxdepth>) PRIMARY KEY, ...other attributes of a tree node... ); To store a word like "stackoverflow": INSERT INTO trie (path) VALUES ('s'), ('st'), ('sta'), ('stac'), ('stack'), ('stacko'), ('stackov'), ('stackove'), ('stackover'), ('stackover'), ('stackoverf

Terribly slow SQL query with COUNT and GROUP BY on two columns

由 不想你离开。 提交于 2019-12-04 11:24:56
I'm archiving this web forum, which normally gets purged about once a week. So I'm screen scraping it, and storing it into my database (PostgreSQL). I also do a little analysis on the data, with some graphs for users to enjoy, like what time of day is the forum most active, and so forth. So I have a posts table, like so: Column | Type ------------+------------------------------ id | integer body | text created_at | timestamp without time zone topic_id | integer user_name | text user_id | integer And I now want to have a post count for each user, for my little top 10 posters table. I came up

Easy way to store and retrieve objects in Java without using a relational DB? [closed]

由 China☆狼群 提交于 2019-12-04 11:22:11
问题 Closed . This question needs to be more focused. It is not currently accepting answers. Want to improve this question? Update the question so it focuses on one problem only by editing this post. Closed 6 years ago . Do you know of an "easy" way to store and retrieve objects in Java without using a relational DB / ORM like Hibernate ? [ Note that I am not considering serialization as-is for this purpose, as it won't allow to retrieve arbitrary objects in the middle of an object graph. Neither
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