create barcode using vb.net Tuple Variables in Java

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8.1 Tuple Variables
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Until now, we have formulated our SQL statements as follows: select ename, init, job from employees where deptno = 20; Actually, this statement is rather incomplete. In this chapter, we must be a little more precise, because the SQL commands are getting slightly more complicated. To be complete and accurate, we should have written this statement as shown in Listing 8.1.
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RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
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Listing 8-1. Using Tuple Variables in a Query select e.ename, e.init, e.job from employees e where e.deptno = 20; In this example, e is a tuple variable. Tuple is just a dignified term for row, derived from the relational theory. In Oracle, tuple variables are referred to as table aliases (which is actually rather confusing), and the ANSI/ISO standard talks about correlation names. Note the syntax in Listing 8-1: You declare the tuple variable in the FROM clause, immediately following the table name, separated by white space only. A tuple variable always ranges over a table, or a table expression. In other words, in the example in Listing 8-1, e is a variable representing one row from the EMPLOYEES table at any time. Within the context of a specific row, you can refer to specific column (or attribute) values, as shown in the SELECT and WHERE clauses of the example in Listing 8-1. The tuple variable precedes the column name, separated by a period. Figure 8-1 shows the column reference e.JOB and its value ADMIN for employee 7900.
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Figure 8-1. The EMPLOYEES table with a tuple variable Do you remember those old-fashioned calendars with one page per month, with a transparent strip that could move up and down to select a certain week, and a little window that could move on that strip from the left to the right to select a specific day of the month If not, Figure 8-2 shows an example of such a calendar. The transparent strip would be the tuple variable in that metaphor. Using the concept of tuple variables, we can describe the execution of the SQL command in Listing 8-1 as follows: 1. The tuple variable e ranges (row by row) over the EMPLOYEES table (the row order is irrelevant).
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RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
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2. 3.
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Each row e is checked against the WHERE clause, and it is passed to an intermediate result set if the WHERE clause evaluates to TRUE. For each row in the intermediate result set, the expressions in the SELECT clause are evaluated to produce the final query result.
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Figure 8-2. Calendar with sliding day indicator window As long as you are writing simple queries (as we have done so far in this book), you don t need to worry about tuple variables. The Oracle DBMS understands your SQL intentions anyway. However, as soon as your SQL statements become more complicated, it might be wise (or even mandatory) to start using tuple variables. Tuple variables always have at least one advantage: they enhance the readability and maintainability of your SQL code.
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You can specify multiple tables in the FROM component of a query. We start this section with an intended mistake, to evoke an Oracle error message. See what happens in Listing 8-2 where our intention is to discover which employees belong to which departments. Listing 8-2. Ambiguously Defined Columns select deptno, location, ename, init from employees, departments; select deptno, location, ename, init * ERROR at line 1: ORA-00918: column ambiguously defined
RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
The message, including the asterisk (*), reveals the problem here. The Oracle DBMS cannot figure out which DEPTNO column we are referring to. Both tables mentioned in the FROM clause have a DEPTNO column, and that s why we get an error message.
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