free barcode generator using vb.net Figure 6-2: The tabular structure of SQL query results in Software

Paint QR Code 2d barcode in Software Figure 6-2: The tabular structure of SQL query results

Figure 6-2: The tabular structure of SQL query results
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List the names, offices, and hire dates of all salespeople. SELECT NAME, REP_OFFICE, HIRE_DATE FROM SALESREPS NAME --------------Bill Adams Mary Jones Sue Smith Sam Clark Bob Smith Dan Roberts Tom Snyder Larry Fitch Paul Cruz Nancy Angelli REP_OFFICE ---------13 11 21 11 12 12 NULL 21 12 22 HIRE_DATE --------12-FEB-88 12-OCT-89 10-DEC-86 14-JUN-88 19-MAY-87 20-OCT-86 13-JAN-90 12-OCT-89 01-MAR-87 14-NOV-88
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In contrast, the following query produces a single row because only one salesperson has the requested employee number. Even though this single row of query results looks less "tabular" than the multi-row results, SQL still considers it to be a table of three columns and one row. What are the name, quota, and sales of employee number 107 SELECT NAME, QUOTA, SALES FROM SALESREPS WHERE EMPL_NUM = 107
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NAME -------------Nancy Angelli
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QUOTA ---------$300,000.00
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SALES ----------$186,042.00
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In some cases the query results can be a single value, as in the following example: What are the average sales of our salespeople SELECT AVG(SALES) FROM SALESREPS AVG(SALES) -----------$289,353.20 These query results are still a table, although it's a very small one consisting of one column and one row. Finally, it's possible for a query to produce zero rows of query results, as in this example: List the name and hire date of anyone with sales over $500,000. SELECT NAME, HIRE_DATE FROM SALESREPS WHERE SALES > 500000.00 NAME ----------HIRE_DATE ---------
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Even in this situation, the query results are still a table. This one is an empty table with two columns and zero rows. Note that SQL's support for missing data extends to query results as well. If a data item in the database has a NULL value, the NULL value appears in the query results when the data item is retrieved. For example, the SALESREPS table contains NULL values in its QUOTA and MANAGER columns. The following query returns these NULL values in the second and third columns of query results: List the salespeople, their quotas, and their managers. SELECT NAME, QUOTA, MANAGER FROM SALESREPS NAME -------------Bill Adams Mary Jones Sue Smith Sam Clark Bob Smith Dan Roberts Tom Snyder Larry Fitch QUOTA -----------$350,000.00 $300,000.00 $350,000.00 $275,000.00 $200,000.00 $300,000.00 NULL $350,000.00 MANAGER ---------104 106 108 NULL 106 104 101 106
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Paul Cruz Nancy Angelli
$275,000.00 $300,000.00
104 108
The fact that a SQL query always produces a table of data is very important. It means that the query results can be stored back into the database as a table. It means that the results of two similar queries can be combined to form a larger table of query results. Finally, it means that the query results can themselves be the target of further queries. A relational database's tabular structure thus has a very synergistic relationship with the relational query facilities of SQL. Tables can be queried, and queries produce tables.
Simple Queries
The simplest SQL queries request columns of data from a single table in the database. For example, this query requests three columns from the OFFICES table: List the location, region, and sales of each sales office. SELECT CITY, REGION, SALES FROM OFFICES CITY -------------Denver New York Chicago Atlanta Los Angeles REGION ------Western Eastern Eastern Eastern Western SALES -----------$186,042.00 $692,637.00 $735,042.00 $367,911.00 $835,915.00
The SELECT statement for simple queries like this one includes only the two required clauses. The SELECT clause names the requested columns; the FROM clause names the table that contains them. Conceptually, SQL processes the query by going through the table named in the FROM clause, one row at a time, as shown in Figure 6-3. For each row, SQL takes the values of the columns requested in the select list and produces a single row of query results. The query results thus contain one row of data for each row in the table.
Figure 6-3: Simple query processing (no WHERE clause)
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