barcode generator in vb.net codeproject Using Correlation Names in Software

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Using Correlation Names
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As I stated earlier, you must qualify your column references by adding table names to those columns that share a name. However, as a general policy, it s a good idea to always qualify column references when joining tables, whether or not it s necessary. This makes referencing the code at a later time much easier if the statement is fully self-documented. However, as your queries become more complex, it can become increasingly tedious to reenter table names every time you reference a column. Because of this, SQL supports correlation names, or aliases, that can be used for the duration of a statement. A correlation name is simply a shortened version of the actual table name that is used to simplify code and make it more readable.
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Take, for example, the last SELECT statement that we looked at. You can recast this statement by using correlation names for the two tables:
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SELECT FROM WHERE AND c.CD_NAME, p.PERF_NAME, c.IN_STOCK CD_INVENTORY AS c, PERFORMERS AS p c.PERF_ID = p.PERF_ID c.IN_STOCK < 15;
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The SELECT statement produces exactly the same results as the preceding statement, only now the tables are referenced by different names, except in the FROM clause. In fact, you use the FROM clause to define the aliases that are used in the rest of the statement. In this case, the CD_INVENTORY table is renamed c, and the PERFORMERS table is renamed p. As a result, c and p must be used everywhere else in the SELECT statement when referring to those tables. Once a correlation name has been defined, you cannot use the actual table name. And yes, this can be confusing because you use the alias in the SELECT clause, but yet it is not defined until the FROM clause that follows the SELECT clause. However, it makes sense when you recall that the FROM clause is always processed first. To better understand how the renaming process works, let s revisit the issue of how SELECT statements are processed. As you might recall from 7, the FROM clause is processed first and the SELECT clause is processed last. That is why the correlation names are defined in the FROM clause. Once they are defined, all other clauses can (and must) use those aliases when qualifying column references. The correlation names are used throughout the remainder of the statement, but they only apply to the statement in which they are defined. If you create a new SELECT statement, you must redefine those names in order to use them in the new statement. As you can see in the previous SELECT statement, a correlation name is defined immediately after the actual table name. The new name follows the AS keyword. However, the AS keyword is not required. In most implementations, you can also use the following convention to rename the tables within a query:
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SELECT FROM WHERE AND c.CD_NAME, p.PERF_NAME, c.IN_STOCK CD_INVENTORY c, PERFORMERS p c.PERF_ID = p.PERF_ID c.IN_STOCK < 15;
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Notice that only the new name is specified, without the AS keyword. This makes the SQL statement that much simpler. In fact, some implementations, such as Oracle, do not allow you to use the AS keyword at all, even though it is part of the SQL standard. Again, this last SELECT statement will provide the same query results that you saw in the two previous examples. Only the statement itself has been changed.
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Creating Joins with More than Two Tables
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Up to this point, the examples that we ve looked at have joined only two tables. However, you can use a comma-separated join to display data from more than two tables. If you refer again to Figure 11-1, you ll see that the PERF_TYPE table is included in the illustration.
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