t any non-key attribute – C D or E being dependent on the primary key (A and B) in our table here.
Partial functional dependency, on the other hand, is another corollary of the above, which states that all non-key attributes – C D or E – if dependent on the subset of the primary key (A and B) and not on it as a whole.
Example :
———-
Fully Functional Dependent : C D E –> A B
Partial Functional dependency : C –> A, D E –> B
Hope that helps!
10. What is a “trigger”
A trigger is a database object directly associated with a particular table. It fires whenever a specific statement/type of statement is issued against that table. The types of statements are insert, update, delete and query statements. Basically, trigger is a set of SQL statements that execute in response to a data modification/retrieva l event on a table.
Other than table triggers there are also schema and database triggers. These can be made to fire when new objects are created, when a user logs in, when the database shutdown etc. Table level triggers can be classified into row and statement level triggers and those can be further broken down into before and after triggers. Before triggers can modify data.
11. What is “index covering” of a query
A nonclustered index that includes (or covers) all columns used in a query is called a covering index. When SQL server can use a nonclustered index to resolve the query, it will prefer to scan the index rather than the table, which typically takes fewer data pages. If your query uses only columns included in the index, then SQL server may scan this index to produce the desired output.
12. What is a SQL view
View is a precomplied SQL query which is used to select data from one or more tables. A view is like a table but it doesn’t physically take any space. View is a good way to present data in a particular format if you use that query quite often.
View can also be used to restrict users from accessing the tables directly.
A view otherwise known as a virtual table is a mere window over the base tables in the database. This helps us gain a couple of advantages:
1) Inherent security exposing only the data that is needed to be shown to the end user
2) Views are updateable based on certain conditions. For example, updates can only be directed to one underlying table of the view. After modification if the rows or columns don’t comply with the conditions that the view was created with, those rows disappear from the view. You could use the CHECK OPTION with the view definition, to make sure that any updates to make the rows invalid will not be permitted to run.
3) Views are not materialized (given a physical structure) in a database. Each time a view is queried the definition stored in the database is run against the base tables to retrieve the data. One exception to this is to create a clustered index on the view to make it persistent in the database. Once you create a clustered index on the view, you can create any number of non-clustered indexes on the view.
13. 存储过程和函数的区别
存储过程是用户定义的一系列sql语句的集合，涉及特定表或其它对象的任务，用户可以调用存储过程，而函数通常是数据库已定义的方法，它接收参数并返回某种类型的值并且不涉及特定用户表。
14. 事务是什么
事务是作为一个逻辑单元执行的一系列操作，一个逻辑工作单元必须有四个属性，称为 ACID(原子性、一致性、隔离性和持久性)属性，只有这样才能成为一个事务:
原子性：事务必须是原子工作单元;对于其数据修改，要么全都执行，要么全都不执行。
一致性：事务在完成时，必须使所有的数据都保持一致状态。在相关数据库中，所有规则都必须应用于事务的修改，以保持所有数据的完整性。事务结束时，所有的内部数据结构(如 B 树索引或双向链表)都必须是正确的。
隔离性：由并发事务所作的修改必须与任何其它并发事务所作的修改隔离。事务查看数据时数据所处的状态，要么是另一并发事务修改它之前的状态，要么是另一事务修改它之后的状态，事务不会查看中间状态的数据。这称为可串行性，因为它能够重新装载起始数据，并且重播一系列事务，以使数据结束时的状态与原始事务执行的状态相同。
持久性：事务完成之后，它对于系统的影响是永久性的。该修改即使出现系统故障也将一直保持。
15. 游标的作用 如何知道游标已经到了最后
游标用 |