barcode generator in vb.net code project Using Predicates in Software

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Using Predicates
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The NULL predicate is very straightforward to implement. Used in conjunction with the IS keyword, the predicate is added to a WHERE clause in the same way any other predicate is added, and it applies only to null values that might exist in the column that you query. The best way to illustrate this is through the use of examples. In these examples, I use the ARTISTS_ BIO table, shown in Figure 9-2. The first example is a SELECT statement that returns rows with a PLACE_OF_BIRTH value of null:
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SELECT * FROM ARTISTS_BIO WHERE PLACE_OF_BIRTH IS NULL;
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The statement returns all columns from the ARTISTS_BIO table; however, it returns only two rows, as you can see in the following query results (your results may look different depending on how your SQL client application displays null values):
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PERFORMER_NAME ---------------William Ackerman Bing Crosby PLACE_OF_BIRTH -------------NULL NULL YEAR_BORN --------NULL 1904
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The fact that the YEAR_BORN column contains a null value for the William Ackerman row has no bearing on the fact that a NULL predicate is used. The NULL predicate in this case identifies the PLACE_OF_BIRTH column only, not the YEAR_BORN column. You can, however, replace the PLACE_OF_BIRTH column in the predicate with the YEAR_BORN column, in which case the rows returned will be those with a YEAR_BORN value of null.
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PERFORMER_NAME: PLACE_OF_BIRTH: VARCHAR(60) VARCHAR(60) Jennifer Warnes Joni Mitchell William Ackerman Kitaro Bing Crosby Patsy Cline Jose Carreras Luciano Pavarotti Placido Domingo Seattle, Washington, USA Fort MacLeod, Alberta, Canada NULL Toyohashi, Japan NULL Winchester, Virginia, United States Barcelona, Spain Modena, Italy Madrid, Spain YEAR_BORN: INT 1947 1943 NULL NULL 1904 1932 NULL 1935 1941
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Figure 9-2
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Returning null values from the ARTISTS_BIO table
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According to the SQL:2006 standard, you can also specify both columns in the NULL predicate, as shown in the following example:
SELECT * FROM ARTISTS_BIO WHERE (PLACE_OF_BIRTH, YEAR_BORN) IS NULL;
When you include both columns, the PLACE_OF_BIRTH column and YEAR_BORN column must both return null values in order for a row to be returned, which in the case of the ARTISTS_BIO table would only be one row.
NOTE
Although the SQL standard permits you to specify multiple columns in the NULL predicate, many implementations do not support this. Instead you must specify two NULL predicates connected with the AND keyword.
As an alternative to including both columns in one predicate, you can write your SELECT statement as follows:
SELECT FROM WHERE AND * ARTISTS_BIO PLACE_OF_BIRTH IS NULL YEAR_BORN IS NULL;
If you execute this statement, you ll receive the following query results:
PERFORMER_NAME ---------------William Ackerman PLACE_OF_BIRTH -------------NULL YEAR_BORN --------NULL
SQL supports another feature in the NULL predicate. You can use the NOT keyword to find the inverse results of the predicate. For example, suppose you want to return all rows that include an actual value in the PLACE_OF_BIRTH column, rather than a null value. Your statement might look like the following:
SELECT * FROM ARTISTS_BIO WHERE PLACE_OF_BIRTH IS NOT NULL;
Your query results will now include seven rows, all of which contain values in the PLACE_ OF_BIRTH column:
PERFORMER_NAME ----------------Jennifer Warnes Joni Mitchell Kitaro Patsy Cline PLACE_OF_BIRTH ----------------------------Seattle, Washington, USA Fort MacLeod, Alberta, Canada Toyohashi, Japan Winchester, Virginia, USA YEAR_BORN --------1947 1943 NULL 1932
9:
Using Predicates
Jose Carreras Luciano Pavarotti Placido Domingo
Barcelona, Spain Modena, Italy Madrid, Spain
NULL 1935 1941
Notice that null values can still exist in other columns. Because only the PLACE_OF_BIRTH column is specified in the NULL predicate, only that column must contain a value in order for a row to be returned. As with the predicates we looked at earlier in the chapter, you can combine the NULL predicate with other types of predicates. For example, you can modify the last example to limit the YEAR_BORN values to certain years, as shown in the following example:
SELECT FROM WHERE AND * ARTISTS_BIO PLACE_OF_BIRTH IS NOT NULL YEAR_BORN > 1940;
Now any rows returned must include a value in the PLACE_OF_BIRTH column and the YEAR_BORN value must be greater than 1940. If you execute this query, you ll receive the following results:
PERFORMER_NAME --------------Jennifer Warnes Joni Mitchell Placido Domingo PLACE_OF_BIRTH ----------------------------Seattle, Washington, USA Fort MacLeod, Alberta, Canada Madrid, Spain YEAR_BORN --------1947 1943 1941
As you can see, only three rows are returned. No rows with a PLACE_OF_BIRTH value of null are returned because null evaluates to unknown, and only WHERE clauses that evaluate to true can be included in the query results.
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