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Subqueries and Query Expressions
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There are many other ways in which this query could be written. The entire query could be written as one top-level grouped query that joins the CUSTOMER and ORDERS table. The join operation could be made explicit with a JOIN operator, and then the results of the join could be grouped in the top-level query. As this example shows, one of the benefits of the SQL query expression capabilities is that they typically provide several different ways to obtain the same query results. The general philosophy behind the capabilities in this area is that SQL should provide the flexibility to express a query in the most natural form. The underlying DBMS must be able to take the query, however expressed, break it down into its fundamentals, and then determine the most efficient way to carry out the query. This internal query execution plan may be quite different from the apparent plan called for by the actual SQL statement, but as long as it produces the same query results, the net effect is to shift the optimization workload from the human user or programmer to the DBMS.
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SQL Queries: A Final Summary
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This concludes the discussion of the SQL queries and the SELECT statement that began in 6. As described in s 6 through 9, the clauses of the SELECT statement provide a powerful, flexible set of capabilities for retrieving data from the database. Each clause plays a specific role in data retrieval: The FROM clause specifies the source tables that contribute data to the query results. Every column name in the body of the SELECT statement must unambiguously identify a column from one of these tables, or it must be an outer reference to a column from a source table of an outer query. The WHERE clause, if present, selects individual combinations of rows from the source tables to participate in the query results. Subqueries in the WHERE clause are evaluated for each individual row. The GROUP BY clause, if present, groups the individual rows selected by the WHERE clause into row groups. The HAVING clause, if present, selects row groups to participate in the query results. Subqueries in the HAVING clause are evaluated for each row group. The SELECT clause determines which data values actually appear as columns in the final query results. The DISTINCT keyword, if present, eliminates duplicate rows of query results. The UNION operator, if present, merges the query results produced by individual SELECT statements into a single set of query results. The ORDER BY clause, if present, sorts the final query results based on one or more columns. The SQL query expression capabilities add row-valued and table-valued expressions and INTERSECT and EXCEPT operations to the SQL1 capabilities. The fundamental flow of query processing is not changed, but the capability to express queries within queries is greatly enhanced.
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QL is not only a query language, but it s also a complete language for retrieving and modifying data in a database. s 10 12 focus on database updates. 10 describes SQL statements that add data to a database, remove data from a database, and modify existing database data. 11 describes how SQL maintains the integrity of stored data when the data is modified. 12 describes the SQL transaction-processing features that support concurrent database updates by many different users.
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CHAPTER 10 Database Updates CHAPTER 11 Data Integrity CHAPTER 12 Transaction Processing
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CHAPTER
Database Updates
QL is a complete data manipulation language that is used not only for database queries, but also to modify and update data in the database. Compared with the complexity of the SELECT statement, which supports SQL queries, the SQL statements that modify database contents are extremely simple. However, database updates pose some challenges for a DBMS beyond those presented by database queries. The DBMS must protect the integrity of stored data during changes, ensuring that only valid data is introduced into the database, and that the database remains self-consistent, even in the event of system failures. The DBMS must also coordinate simultaneous updates by multiple users, ensuring that the users and their changes do not interfere with one another. This chapter describes the three SQL statements that are used to modify the contents of a database: INSERT Adds new rows of data to a table DELETE UPDATE Removes rows of data from a table Modifies existing data in the database
In 11, SQL facilities for maintaining data integrity are described. 12 covers SQL support for multiuser concurrency.
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