In MySQL, you can specify a column as auto_increment, the value will increment 1 at each insert. SQL Server also provide similar thing. Oracle using SEQUENCE and NEXTVAL to do the job.
Create a SEQUENCE:
Oracle provide PL/SQL packages let you extend the database functionality . You can write you own package or utilize the packages shipped with Oracle release.Oracle supplies all kinds of packages, familiar with these packages is a must-have skill of DBA.
Packages is a collection of different program objects include procedures, functions, variables, constants, cursors, exceptions etc.
enum(enumeration) is a user-defined type consisting of a set of enumerators( enumerator --- named integer constant)
Simple enum definition:
When use generate lexer with flex and compile it as cplusplus file, you will get an error like this:
But when compile as c file it will be no problem, if you see the generated c file , you will see:
I always have the question: How the interface find its implementation ? In the calling code we always declare object with the interface name, Never need to declare the implementation class. We never explicitly tell the compiler which implementation to use.
The answer is , compiler don't know which implementation to use. When you declare a interface , there is no instance created, you have to new something and assign that instance to the interface. And the instance can be anything that implement the interface.The answer is , compiler don't know which implementation to use. When you declare a interface , there is no .
In java extends declaration syntax we often see something like this:
The question mark also show up on other place, inside an angle bracket. It means any class, its a wildcard. Its the Generic of Java. It called bounded wildcard.
In Oracle, the dual table is a special kind of one-row and one-column database table. dual is a table which is created by oracle along with the data dictionary. It consists of exactly one column whose name is dummy and one record. The value of that record is X. Note that the outer query references the dummy table .
Oracle released its 11g Database, it introduced a number of new features and improvement for PL/SQL.
A new built-in function REGEXP_COUNT available in PLSQL of Oracle11g. It match a regular expression pattern and return the number of matches. Here is an example: