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Database Programming
Databases are collections of tables that maintain and display information, often collaboratively; this information can be used for interaction with an application or gaining general knowledge. Questions about database engines and modifying or using them belong in this category.
8,803 Questions
How do you select all columns of all rows from a table?
You select all columns of all rows from a table with the select * from table_name sql statement. Be careful, this can potentially be a very expensive, poor performance, network intensive type of operation - it is better to select only the columns and rows needed.
How is indexing faster than sorting when accessing data from a table?
Indexing can be faster than sorting. It can also be slower. It depends on how many rows your predicate clause is selecting, and on the data dispersion represented by those index keys. If you are selecting a few rows, such as less than 4% of the table, then by all means use an index. If you are selecting a lot of rows, such as more than 25% of the table, then the use of the index will usually degrade performance, so go for the full table scan followed by a sort. (Of course, now you are adding the use of sort space, so that is a consideration.) The break-even point depends on the particular RDBMS and on the structure of the data. If your RDBMS supports EXPLAIN PLAN, then use it, and learn to know what it means. Also, make sure you understand your data, because (sometimes) only you know the best way to query it, and it might make sense to override the optimzer and force a certain execution plan based on your knowledge.
SQLite is an ACID-compliant embedded relational database management system contained in a relatively small (~225 KB[1]) C programming library. The source code for SQLite is in the public domain. Unlike client-server database management systems, the SQLite engine is not a standalone process with which the program communicates. Instead, the SQLite library is linked in and thus becomes an integral part of the program. It can also be called dynamically. The program uses SQLite's functionality through simple function calls, which reduces latency in database access as function calls within a single process are more efficient than inter-process communication. The entire database (definitions, tables, indices, and the data itself) is stored as a single cross-platform file on a host machine. This simple design is achieved by locking the entire database file at the beginning of a transaction. SQLite is embedded into a growing number of popular applications. For example, Mozilla Firefox stores a variety of configuration data, (bookmarks, cookies, etc.), in internally managed SQLite databases. As another example, Google's Android OS for cellphones and other small devices includes SQLite.
Who are actors on the scene in DBMS?
SOURCE: http://jeenuutreja.blogspot.in/2012/07/actors-on-scene-in-dbms-in-this-topic.html
I'd copy paste this here but these guys have earned a visit!
What is Difference between uncontrolled events and controlled events?
a controlled event is a variable that you can manipulate and change or do anything you want with you want. uncontrolled event is something you cannot help. in example, if you were doing something with eye colors, you just have to deal with the eye color the person has. you cannot change it.
Explain the difference between a heterarchical network and a hierarchical network?
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What does a 6633N SAP 10-50 CCMS mean?
6633N SAP 10-50 GC/MS is a Quest Labs designation for a standard urinalysis which tests for cannabinoids, amphetamines, opiates etc.
There is a 50ng/ml drug/metabolite cutoff level for the initial immunoassay screening with a 15ng/ml confirmation cutoff using Gas Chromotography/Mass Spectrometry (GC/MS) method.
Only specimens which fail the initial immunoassay screening, referred to as "presumptive positives", are sent for GC/MS confirmation at the more sensitive level.
During the initial immunoassay screening, tests for validity such as specific gravity, creatinine level, alduterant (additive) checks etc. nare performed to ensure that the specimen has not been tampered with and is actual human urine.
If a specimen passes intitial screening a "pass" is reported and no further testing is performed.
These are collectively referred to as "physical capital" or "tangible assets." This includes resources such as buildings, machinery, technology, and tools that contribute to the production of goods and services. In contrast, "human capital" refers to the skills, knowledge, and experience of individuals that also contribute to the productivity and success of an organization.
A DBMS is typically shared among many users. Transactions from these users
can be interleaved to improve the execution time of users' queries. By interleaving
queries, users do not have to wait for other user's transactions to complete
fully before their own transaction begins. Without interleaving, if user A begins
a transaction that will take 10 seconds to complete, and user B wants to begin
a transaction, user B would have to wait an additional 10 seconds for user
A's transaction to complete before the database would begin processing user B's
request.
A database is important because it stores a lot of information. This information can later be used in cases of emergency or reference.
Problems that occur when incorrect data is stored?
There are quite a few problems with storing data
1) security issues
2) excessive data
Explain the concepts of recoverable schedule in dbms?
Transactions commit only after all transactions whose changes they read, commit
Example of multiple relationship between entities?
bascially, i dont know thr answer myself. so i am waiting for someone to upload it. hurry yeah, i need to finish my assignment off.
thank you
What are limitations of object oriented database model?
- Schema Changes: In an RDBMS modifying the database schema either by creating, updating or deleting tables is typically independent of the actual application. In an OODBMS based application modifying the schema by creating, updating or modifying a persistent class typically means that changes have to be made to the other classes in the application that interact with instances of that class. This typically means that all schema changes in an OODBMS will involve a system wide recompile. Also updating all the instance objects within the database can take an extended period of time depending on the size of the database.
- Language Dependence: An OODBMS is typically tied to a specific language via a specific API. This means that data in an OODBMS is typically only accessible from a specific language using a specific API, which is typically not the case with an RDBMS.
- Lack of Ad-Hoc Queries: In an RDBMS, the relational nature of the data allows one to construct ad-hoc queries where new tables are created from joining existing tables then querying them. Since it is currently not possible to duplicate the semantics of joining two tables by "joining" two classes then there is a loss of flexibility with an OODBMS. Thus the queries that can be performed on the data in an OODBMS is highly dependent on the design of the system.
What does record mean in relation to databases?
it when you see your first name/ last name, date of birth school if you think about it you have your first name in one column and DOB (date of birth) in one and last name in the other it is set. It is just a database which is filled with important or
essential information
hope that helps so good look if your learning about databases or for homework.
Difference between dB dBm and dBi?
dB or dBW relativ to 1W
: dBm for dB relativ to 1 milliwatt. 0 dBm= 1 mW = -30dBW
.
Its interesting,but these terms are used interchangeably at times, erroneously.The term dBm is used by
communications engineers and it is absolute.Most Power meters commercially available have this scale.
It is : dBm=10log(base 10)(P1/P2) at two different Power points.Power is read always across a 50 Ohm
resistor.IEEE has made this a standard and 1mW=0dBm(-40dBm is 100nW accross 50 Ohm and +20dBm is 100mW).
As far as I know dB(not DB)refers simly to gain(and loss/attenuation as -dB).It is different this time
as the equation is not Power but a simple ratio: dB=20log(base 10)(Gain or attenuation).One can have
a reference and above that reference he is talking in positive dBs(Gain) and below in negative dBs(loss
or attenuation).Such scales can then be modified as we have done for Acoustic Emission where we talk in
dBae and our refence is the "perfect" sensor giving us ONLY 1microVolt(!) Peak output noise.Anyway a
good goal.So for 40dBae a sensor/Amplifier(40dB Gain)output across a 50 Ohm resistor is 10mV.
Differences between hand file and flat file?
The original answer was "difference between flat file and hand file". Not an answer, only repeats the question.
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[ This question seems to have been placed, originally, in the wrong category: Database Programming. It contains the term "flat file", which does make it seem to be a computer item, but flat file can also refer to a type of hand tool. The fact that the question deals with the difference between a hand file and a flat file, two varieties of a common hand tool, makes it clear that this question should be in a category addressing tools in general, and hand tools in particular.
It may be that the person asking the question thought "hand file" referred to a type of computer storage file, and thus placed the question in what he thought was the appropriate category. My answer addresses the two terms as they relate to metal or wood working, so I copied the question (and answer) to the only existing category that fits: Building and Carpentry. I wish there were a separate category under Home and Garden for just "Tools", but there isn't, so I chose Building and Carpentry. In doing so, I may be just confusing the issue. I hope I didn't, and if I did, I'm sorry. OffGrid9 ]
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A hand file is rectangular in cross section, parallel in both width and thickness from end to end. One edge (the "safe edge" has no teeth, allowing a user to file one surface right up to the edge in a corner without marring the other surface. In recent years, it is common to find hand files that taper in thickness, but the width is still parallel.
A flat file is similar to a hand file. It is also rectangular in cross section, but it tapers in both thickness and width, and it does not have a safe edge. In older flat files, the width was usually parallel for most of the file's length, then curved near the end to taper in. On modern flat files, the width usually tapers in throughout the file's length - the sides may curve, the taper increasing towards the end, or the sides may be straight, with the taper being continuous from end to end.
Advantages of using indexes in database?
Advantages of an index over no index
If no index exists on a table, a table scan must be performed for each table referenced in a database query. The larger the table, the longer a table scan takes because a table scan requires each table row to be accessed sequentially. Although a table scan might be more efficient for a complex query that requires most of the rows in a table, for a query that returns only some table rows an index scan can access table rows more efficiently.
The optimizer chooses an index scan if the index columns are referenced in the SELECT statement and if the optimizer estimates that an index scan will be faster than a table scan. Index files generally are smaller and require less time to read than an entire table, particularly as tables grow larger. In addition, the entire index may not need to be scanned. The predicates that are applied to the index reduce the number of rows to be read from the data pages.
7 If an ordering requirement on the output can be 7 matched with an index column, then scanning the index in column order will 7 allow the rows to be retrieved in the correct order without a sort.
Each index entry contains a search-key value and a pointer to the row containing that value. If you specify the ALLOW REVERSE SCANS parameter in the CREATE INDEX statement, the values can be searched in both ascending and descending order. It is therefore possible to bracket the search, given the right predicate. An index can also be used to obtain rows in an ordered sequence, eliminating the need for the database manager to sort the rows after they are read from the table.
In addition to the search-key value and row pointer, an index can contain include columns, which are non-indexed columns in the indexed row. Such columns might make it possible for the optimizer to get required information only from the index, without accessing the table itself.
Note:The existence of an index on the table being queried does not guarantee an ordered result set. Only an ORDER BY clause ensures the order of a result set.
What does it mean When your internal antivirus database is out of date?
That the virus definition of your anti-virus software need to be updated.
What is the foreigh key in DBMS explainwith example?
In the context of relational databases, a foreign key is a referential constraint between two tables.[1] The foreign key identifies a column or a set of columns in one (referencing) table that refers to a column or set of columns in another (referenced) table. The columns in the referencing table must be the primary key or other candidate key in the referenced table. A table may have multiple foreign keys, and each foreign key can have a different referenced table. Each foreign key is enforced independently by the database system. Therefore, cascading relationships between tables can be established using foreign keys. Improper foreign key/primary key relationships or not enforcing those relationships are often the source of many database and data modeling problems.
