2.Is a primary key constraint a special kind of table constraint or database constraint?
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According to me it is a table constraint as we can enforce primary key on table not whole database you might thinking that it is used to make a Field global in database but the real thing is that it give a unique identity to a particular tuple.
30843
Well your question is not clear.But if you wanna know what a database system is ...\nA database keeps all possible information in the form of records that are normally maintained in the form of tables.\nThere are some really advance techniques of handling these databases using Oracle or SQL server or DB2 etc.\nIf you want to do some basic operations you'll have to learn the language called SQL(Structured Query Language) which is language for the manipulation of databases.\nThese systems are normally the backend support for systems whose front ends are visible to you in the form various Web Applications,Web services and other Desktop based applications.
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As far as I know so far mysql doesn't support declearing the foreign keys like oracle, but you don't have to worry about that, keep the field of the table the way it is and carefully make the query. Keep the names of both of the table's field that you want to make foreign keys, and in this way when you'll make the query you'll see that the queries are same as the oracle but you don't have to declear them as foreign keys. For examples you can contact me.
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You have to be sure that you set the constraint on a collumn that is "not null"\n\nSet first the collumn on "not null"
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The file structure looks pretty close to what you would want.  Have a table with EmployeeId (PK), ManagerId (nullable), name, etc etc..  Then create a check constraint on the ManagerId field to ensure that a value exists inside of employees table already (except when null).  If you then want to have two different queryable objects you could create a view like ManagerEmployeeView and PeonEmployeeView =)
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It is a database software.
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Some *very* general steps for a simple database:\n\nDecide the necessary fields and their parameters\nDetermine a primary key field\nDesign appropriate queries\n\nNow, there are certain considerations to make when determining data types - for instance, if you have a numerical field, do you want to store the values as integers or as "double"? One may use less space than the other, where the other will allow for the precision and range you may need.
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You ask a very good quesion.  I will try my best to answer.\n\nOne definition of entity is something that exists (living or non-living).  An ERD is a high-level abstraction.  A table on the other hand, is a coded structure within an RDBMS.\n\nThinking about it in terms of modeling, an actual abstracted entity, might be represented by one or more implemented tables.  For instance, We might have two ERD entities "Customer" and "Product", but in an actual normalized implementation, "Customer" might in practice consist of two very different tables like "Customer-Corporation" and "Customer-Human".\n\nFrom a strict database perspective I suppose your term is more precise.  From a philosophical perspective, entities/tables are unchanging, it is their relationships that give them life.\n\nHope this helps.
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A Deadlock is mostly referenced to the SQL Deadlock.\n\nA Deadlock can be summarized in simple terms - that as a situation when a row or rows of records are temporarily read-locked by a select statement and the same time another update/delete statement is trying to execute on the same row/rows.  this results in a Deadlock. This prevents the second query from executing.\n\nHowever, deadlocks can be prevented/eliminated by optimizing the front-end coding and sequence of events.  Still if a deadlock occurs, then a "requery" is needed.
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It's a synchronized database distributed accross multiple servers. \n\nhttp://en.wikipedia.org/wiki/Distributed_database\n\nGoogle is your friend.
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The central concept of a database is that of a collection of records, or pieces of knowledge. Typically, for a given database, there is a structural description of the type of facts held in that database: this description is known as a schema. The schema describes the objects that are represented in the database, and the relationships among them. There are a number of different ways of organizing a schema, that is, of modeling the database structure: these are known as database models (or data models). The model in most common use today is the relational model, which in layman's terms represents all information in the form of multiple related tables each consisting of rows and columns (the true definition uses mathematical terminology). This model represents relationships by the use of values common to more than one table. Other models such as the hierarchical model and the network model use a more explicit representation of relationships.\n\n\n\n\nhttp://rd13doc.cern.ch/Notes/049/Note049-6.html
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In the database design you can add default constraints on columns to avoid data entry errors but usually the front end application that is used to capture the data does most of the validation and disallows saving of that which means the database is left untouched until the correct information is entered.
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Object Oriented databases may be characterized quite simply; they typically represent database systems that integrate tightly with a language with object-oriented features such as C++, Java, or Smalltalk, that allow programs to link data structures to databases in such a way that the data structures trivially become "persistent."\n\nthus once you tie a data structure to the database, you no longer need to be concerned about whether a piece of data is in memory or not. as soon as your program refers to it, the data will be pulled into memory if it is not already there. As soon as your program updates the data, it will be updated into the database.
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A relation in 3NF with a unique key is always in BCNF because BCNF deals with dependencies among the candidate keys. Since we have only one candidate key over here hence relation is also in BCNF.
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Let's pretend we have a customer table with all our customers and an invoice table with all the invoices we've generated.  Each customer has a unique customer id, and every invoice has a unique invoice id PLUS has the id of the customer it belongs to.\n\nA primary key is usually used to make sure you don't have two records in the same table with the same key value.  If we had the same customer in our table twice, we might try to send them two bills, or do something twice.  The primary key constraint of uniqueness will not allow us to insert a second customer in the table with the same id.\n\nA foreign key is normally used to make sure the parent record IN ANOTHER TABLE exists.  For example, it would not make sense to create an invoice with no customer to send it to.  A foreign key constraint will not let you put a record in the invoice table unless the customer id on it exists in the customer table.
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The tradeoffs are always the same when implementing types or subtypes when creating anything..logical or not, schema or not.....it's all a means to an end that is an effect based on cause...
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Hi .. \n\ni guess the following page can reslove your issue\nhttp://www.phpfreaks.com/postgresqlmanual/