barcode generator in vb.net code project Querying SQL Data in Software

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Querying SQL Data
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Your query results will now contain only two columns, as shown in the following:
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NAME ----------------Jennifer Warnes Joni Mitchell William Ackerman Kitaro Bing Crosby Patsy Cline Jose Carreras Luciano Pavarotti Placido Domingo PLACE_OF_BIRTH -----------------------------Seattle, Washington, USA Fort MacLeod, Alberta, Canada Germany Toyohashi, Japan Tacoma, Washington, USA Winchester, Virginia, USA Barcelona, Spain Modena, Italy Madrid, Spain
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Notice that the name of the first column is NAME, rather than PERFORMER_NAME. This is because the AS subclause (specifying NAME) is defined as part of the PERFORMER_ NAME derived column. If you were to specify the DISTINCT keyword in this particular situation, you would still receive the same number of rows, although they might not be returned in the same order as they were when you didn t use the keyword, depending on the SQL implementation. The reason that the DISTINCT keyword would make no difference in the query results is that there are no duplicate rows in the table. However, using the DISTINCT keyword can affect performance, particularly if your RDBMS has to sort through a large number of rows, so be sure to use the keyword only when necessary. Now let s take a look at an example that uses the DISTINCT keyword. Suppose your database includes a table that matches performers to types of music, as shown in Figure 7-2.
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PERFORMER_NAME: PERFORMER_TYPE: VARCHAR(60) VARCHAR(10) Jennifer Warnes Jennifer Warnes Joni Mitchell Joni Mitchell Joni Mitchell William Ackerman Kitaro Kitaro Folk Pop Pop Folk Jazz New Age New Age International
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Figure 7-2
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The PERFORMER_NAME and PERFORMER_TYPE columns of the PERFORMER_ TYPE table
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SQL: A Beginner s Guide
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If your SELECT statement includes both (all) columns in the SELECT clause, as shown in the following example, your query will return all rows:
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SELECT * FROM PERFORMER_TYPE;
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It does not matter if you specify the DISTINCT keyword in this case because your query results include no duplicate rows. The results would be the same whether you include the ALL keyword, rather than DISTINCT, or whether you specify neither of the two qualifiers. In either case, the query results would include the same information that is shown in the table in Figure 7-2. Now let s take a look at the same statement, only this time it specifies the DISTINCT keyword and only one of the two columns:
SELECT DISTINCT PERFORMER_NAME FROM PERFORMER_TYPE;
Notice that this statement includes only the PERFORMER_NAME column, which includes duplicate values. By using the DISTINCT keyword, your query results will include only one instance of each value. If you execute the SELECT statement in the preceding example, your query results will look similar to the following:
PERFORMER_NAME ------------------------Jennifer Warnes Joni Mitchell Kitaro William Ackerman
Although there are seven rows in the PERFORMER_TYPE table, only four rows are returned because there are only four unique values in the PERFORMER_NAME column and the other values are duplicates. As you can see, the SELECT clause and the FROM clause are fairly straightforward, at least at this level of coding. Once we get into more complex structures, you ll find that both these clauses can at times become more complicated. However, the important thing to remember right now is that these clauses act as the foundation for the rest of the SELECT statement. In terms of execution, the SELECT statement, for all practical purposes, begins with the FROM clause and ends with the SELECT clause. (The ORDER BY clause is used primarily for display purposes and doesn t affect which information is actually returned. The ORDER BY clause is discussed in more detail in the Use the ORDER BY Clause to Sort Query Results section later in this chapter.)
Use the WHERE Clause to Define Search Conditions
The next clause in the SELECT statement is the WHERE clause. The WHERE clause takes the values returned by the FROM clause (in a virtual table) and applies the search condition that is defined within the WHERE clause. The WHERE clause acts as a filter on the results returned by the FROM clause. Each row is evaluated against the search condition. Those rows
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