In the previous video we talked about some of the most popular data types. We are going to discuss them in more detail. The data type we are going to start with is CHAR and NCHAR. I told you of both of these but I never explained the difference. That's because there is some other stuff I need to explain before I can explain the difference. This has to deal with what is known as character sets.
When you have a string, there are only so many characters you are allowed to store in that string. The characters you are allowed to store is determined by what is known as the character set.
A common character set is ASCII. This character set allows you to store English characters, numbers, and some symbols. ASCII started with 127 characters, and then they came out with the ASCII extended, which allows for up to 255 characters. Even with 255 characters though, we are limited in what we can store using one character set. If the computer only allows ASCII, we are going to be limited when working with different languages. Of course it works for some situations, but globalization of software has been a big thing with the development of the interwebs …and the movement towards a new world order (Revelation 13:7).
That means that ASCII is no longer the best character set. It has largely been replaced with a character set known as Unicode. Oracle has a few Unicode character sets that we can use when we work with string data.
When you start studying character sets, I can promise that you will run across the word encoding. Encoding refers to the way that the allowed characters can be stored on the computer. A computer doesn't just store a letter, everything has to be stored in binary. Unicode is the character set, but it has numerous different encodings. Essentially, the computer can store the same characters in multiple different ways, depending on which encoding is used.
The most popular encodings for Unicode are UTF-8 and UTF-16. UTF stands for Unicode Transformation Format. In the next video we will be discussing these in detail and express their differences. Once we got that down, we'll be able to loop back around to data types.
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A trigger is a named PL/SQL unit that is stored in the database and executed ( fired ) in response to a specified event that occurs in the database.
Overview of Triggers.
A trigger is a named program unit that is stored in the database and fired (executed) in response to a specified event. The specified event is associated with either a table, a view, a schema, or the database, and it is one of the following:
A database manipulation (DML) statement ( DELETE , INSERT , or UPDATE )
A database definition (DDL) statement ( CREATE , ALTER , or DROP )
A database operation ( SERVERERROR , LOGON , LOGOFF , STARTUP , or SHUTDOWN )
The trigger is said to be defined on the table, view, schema, or database.
A DML trigger is fired by a DML statement, a DDL trigger is fired by a DDL statement, a DELETE trigger is fired by a DELETE statement, and so on.
An INSTEAD OF trigger is a DML trigger that is defined on a view (not a table). The database fires the INSTEAD OF trigger instead of executing the triggering DML statement. For more information, see Modifying Complex Views (INSTEAD OF Triggers).
A system trigger is defined on a schema or the database. A trigger defined on a schema fires for each event associated with the owner of the schema (the current user). A trigger defined on a database fires for each event associated with all users.
A simple trigger can fire at exactly one of the following timing points :
Before the triggering statement executes.
After the triggering statement executes.
Before each row that the triggering statement affects.
After each row that the triggering statement affects.
A compound trigger can fire at more than one timing point. Compound triggers make it easier to program an approach where you want the actions you implement for the various timing points to share common data. For more information, see Compound Triggers.