barcode generator in vb.net codeproject Creating the Outer Join in Software

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Creating the Outer Join
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As I mentioned earlier in this chapter, an outer join returns all matched rows and some or all unmatched rows, depending on the type of outer join you create. SQL supports three types of outer joins:
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Left Returns all matched rows and all unmatched rows from the left table the table to the left of the JOIN keyword. Right Returns all matched rows and all unmatched rows from the right table the table to the right of the JOIN keyword. Full Returns all matched and unmatched rows from both tables.
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Because it is a relatively new feature, few SQL implementations currently support full outer joins Oracle and SQL Server do, but MySQL does not.
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An outer join follows the same syntax as an inner join, except that, rather than using the INNER JOIN keywords (or just the JOIN keyword), you use LEFT OUTER JOIN, RIGHT OUTER JOIN, or FULL OUTER JOIN. Note that the OUTER keyword is optional. For example, you can specify LEFT JOIN instead of LEFT OUTER JOIN. The best way to illustrate the differences between the types of outer joins is to show you examples of query results from each type. To illustrate the differences, I use the CD_INFO table and the CD_TYPE table, shown in Figure 11-5.
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Accessing Multiple Tables
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CD_INFO TITLE: VARCHAR(60) Famous Blue Raincoat Blue Past Light Out of Africa Fundamental Blues on the Bayou TYPE_ID: CHAR(4) FROK CPOP NEWA STRK NPOP BLUS STOCK: INT 19 28 6 8 10 11
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CD_TYPE TYPE_ID: CHAR(4) FROK CPOP NEWA CTRY STRK BLUS JAZZ TYPE_NAME: CHAR(20) Folk Rock Classic Pop New Age Country Soundtrack Blues Jazz
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Figure 11-5
Joining the CD_INFO and CD_TYPE tables
In the first example, I define an inner join on the two tables, just to show you how the query results would normally look:
SELECT i.TITLE, t.TYPE_NAME, i.STOCK FROM CD_INFO i JOIN CD_TYPE t ON i.TYPE_ID = t.TYPE_ID;
This statement returns the following query results:
TITLE -------------------Famous Blue Raincoat Blue Past Light Out of Africa Blues on the Bayou TYPE_NAME ----------Folk Rock Classic Pop New Age Soundtrack Blues STOCK ----19 28 6 8 11
In most cases, the inner join will provide all the information you need. But suppose you want to include the unmatched rows from the CD_INFO table. In that case, you would create a left outer join, as shown in the following example:
SELECT i.TITLE, t.TYPE_NAME, i.STOCK FROM CD_INFO i LEFT OUTER JOIN CD_TYPE t ON i.TYPE_ID = t.TYPE_ID;
SQL: A Beginner s Guide
Notice that I ve replaced JOIN (for INNER JOIN) with LEFT OUTER JOIN. As I mentioned earlier, you can omit the OUTER keyword in most implementations. If you execute this statement, you ll receive the following query results:
TITLE -------------------Famous Blue Raincoat Blue Past Light Out of Africa Fundamental Blues on the Bayou TYPE_NAME ----------Folk Rock Classic Pop New Age Soundtrack NULL Blues STOCK ----19 28 6 8 10 11
As you may have noticed, the Fundamental row is now included in the query results. Although this row doesn t include matched columns, it is still included in the query results because it is part of the left table. For this row, the TYPE_NAME column is assigned a null value because no logical value can be returned for this column. The null value serves as a placeholder. You can also return the unmatched rows from the CD_TYPE table, which is the table to the right of the JOIN keyword:
SELECT i.TITLE, t.TYPE_NAME, i.STOCK FROM CD_INFO i RIGHT OUTER JOIN CD_TYPE t ON i.TYPE_ID = t.TYPE_ID;
This statement is nearly the same as the preceding statement, except that RIGHT has been specified. The statement returns the following query results:
TITLE -------------------Famous Blue Raincoat Blue Past Light NULL Out of Africa Blues on the Bayou NULL TYPE_NAME ----------Folk Rock Classic Pop New Age Country Soundtrack Blues Jazz STOCK ----19 28 6 NULL 8 11 NULL
This time the unmatched columns from the right table are included in the results, and null values are shown for the TITLE and STOCK column. If you want to return all unmatched rows, you would need to modify the statement to define a full outer join:
SELECT i.TITLE, t.TYPE_NAME, i.STOCK FROM CD_INFO i FULL OUTER JOIN CD_TYPE t ON i.TYPE_ID = t.TYPE_ID;
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