TAILIEUCHUNG - Oracle XSQL- P9

Oracle XSQL- P9Welcome to the exciting world of eXtended Structured Query Language (XSQL) development! What’s so exciting? Efficiency and ease of use. XSQL isn’t some razzle-dazzle technology to wow your users. It also isn’t the latest X standard du jour that no one can stop talking about until you ask, “But what does it do for me today?” The problem with all of the great stuff out there is that no one technology does it all. | 140 Chapter 8 A couple of times you ve used a select statement that has a where clause such as the following SELECT ename sal FROM emp WHERE sal 1000 This statement returns just the names of employees with salaries greater than 1 000. This example has all the component parts. Here s a translation of this select statement in plain English SELECT the data I want FROM one or more data sources WHERE these conditions are met The first component the data I want is called the elements clause. In our first statement you used the wildcard operator to say that you wanted all the fields for all the rows where as in the second statement you asked only for two specific fields. The second component is the target it tells the database what objects will be involved. The last component the where clause is optional as is demonstrated in the first statement above. However you ll almost always use it in real-life code. The first topic covered will actually be the second component the target. You need to know a little bit about this before you can fully understand the element expression which is covered secondly. The section that follows is about the where clause. Target Clause The target clause is typically a comma-delimited list of tables but it can also contain a couple other types of objects. First you ll learn what else can be in the list. Then since you have experience with select statements involving only one table you ll learn what happens when there is more than one object as well as take your first look at joins. Finally you ll learn table aliasing and when it is used. Types of Target Objects So far you ve only seen tables used as target objects. However tables can also contain snapshots and views which you haven t learned about yet. A view acts like a table but is actually a window or view onto the content of one or more tables. A snapshot is a view that refreshes periodically. Usually a snapshot is used when there is a table that you want on a remote database however going

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