barcode generator vb.net download Subqueries and Query Expressions in Software

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Subqueries and Query Expressions
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The top-level query is a straightforward statement of the English-language request it asks for the description and price of those products whose identification (as in previous examples, a manufacturer-ID/product-ID pair) matches some set of products. This is expressed as a subquery set membership test in the WHERE clause. The subquery generates a two-column table of subquery results, which are the identifications of the products that meet the stated order size criterion. It s certainly possible to express this query in other ways. From the discussion in 7, you probably recognize that it can be stated as a join of the PRODUCTS and ORDERS tables with a compound search condition: List the description and price of all products with individual orders over $20,000.
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SELECT FROM WHERE AND AND DISTINCT DESCRIPTION, PRICE PRODUCTS, ORDERS (MFR_ID = MFR) (PRODUCT_ID = PRODUCT) (AMOUNT > 20000.00);
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Note that we had to add the DISTINCT keyword because there are two orders over $20,000 for the Right Hinge product. This is an equally valid statement of the query, but it s a lot further removed from the English-language request, and therefore more difficult to understand for most people. As queries become more complex, the ability to use tablevalued subqueries becomes even more useful to simplify and clarify SQL requests.
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The SQL Query Specification
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The SQL standard formalizes the definition of what we have loosely been calling a SELECT statement or a query in the last three chapters into a basic building block called a query specification. For a complete understanding of the SQL table expression capabilities in the next section, it s useful to understand this formal definition. The form of a SQL query specification is shown in Figure 9-16. Its components should be familiar from the earlier chapters: A select list specifies the columns of query results. Each column is specified by an expression that tells the DBMS how to determine its value. The column can be assigned an optional alias with the AS clause. The keywords ALL or DISTINCT control duplicate-row elimination in the query results. The FROM clause specifies the tables that contribute to the query results. The WHERE clause describes how the DBMS should determine which rows are included in the query results and which should be discarded. The GROUP BY and HAVING clauses together control the grouping of individual query results rows in a grouped query, and the selection of row groups for inclusion or exclusion in the final results. The ORDER BY clause specifies the desired sequence of the rows in the query results.
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* table-name.* value-expression column-name AS ,
FROM
table-specification ,
WHERE search-condition
GROUP BY grouping-column-list
HAVING search-condition
ORDER BY order-column-list
FIGURE 9-16
SQL query speci cation: formal de nition
The query specification is the basic query building block in the SQL standard. Conceptually, it describes the process of combining data from the tables in the FROM clause into a row/column table of query results. The value of the query specification is a table of data. In the simplest case, a SQL query consists of a simple query specification. In a slightly more complex case, a query specification is used to describe a subquery, which appears within another (higher-level) query specification. Finally, query specifications can be combined using table-valued operations to form general-purpose query expressions, as described in the next section.
Query Expressions
The SQL standard defines a query expression as the full, general-purpose way that you can specify a table of query results in SQL. The basic building blocks you can use to create a query expression are the following: A query specification, as described in the preceding section (SELECT FROM ). Its value is a table of query results. A table-value constructor, as previously described (VALUES ). Its value is a table of constructed values. An explicit table reference (TABLE tblname). Its value is the contents of the named table.
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