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As described in the previous section, table aliases are required in queries involving self-joins. However, you can use an alias in any query. For example, if a query refers to another user s table, or if the name of a table is very long, the table name can become tedious to type as a
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column qualifier. This query, which references the BIRTHDAYS table owned by the user named SAM: List names, quotas, and birthdays of salespeople.
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SELECT SALESREPS.NAME, QUOTA, SAM.BIRTHDAYS.BIRTH_DATE FROM SALESREPS, SAM.BIRTHDAYS WHERE SALESREPS.NAME = SAM.BIRTHDAYS.NAME;
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becomes easier to read and type when the aliases S and B are used for the two tables: List names, quotas, and birthdays of salespeople.
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SELECT S.NAME, S.QUOTA, B.BIRTH_DATE FROM SALESREPS S, SAM.BIRTHDAYS B WHERE S.NAME = B.NAME;
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Figure 7-10 shows the basic form of the FROM clause for a multitable SELECT statement, complete with table aliases. The clause has two important functions: The FROM clause identifies all of the tables that contribute data to the query results. Any columns referenced in the SELECT statement must come from one of the tables named in the FROM clause. (There is an exception for outer references contained in a subquery, as described in 9.) The FROM clause specifies the tag that is used to identify the table in qualified column references within the SELECT statement. If a table alias is specified, it becomes the table tag; otherwise, the table s name, exactly as it appears in the FROM clause, becomes the tag. The only requirement for table tags in the FROM clause is that all of the table tags in a given FROM clause must be distinct from each other. Even if you don t use table aliases in SQL queries that you write, you are likely to encounter them if you examine the SQL generated by report-writing or business analysis tools. These tools typically present a graphical interface that allows you to easily choose the columns, tables, matching columns, search conditions, and other elements of your query, and they automatically generate the corresponding SQL statements that are passed to the DBMS. The tool will almost always use table tags (typically using tags like T1, T2, T3, etc.) in the FROM clause of the generated SQL, allowing it to easily and unambiguously specify the rest of the query, regardless of the actual names of the tables, columns, and other database elements. The SQL standard optionally allows the keyword AS to appear between a table name and table alias. It also uses the term correlation name to refer to what we have called a table alias. The function and meaning of a correlation name are exactly as described here;
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FIGURE 7-10
The FROM clause syntax diagram
7:
Multitable Queries (Joins)
many SQL products use the term alias, and it is more descriptive of the function that a table alias performs. The SQL standard specifies a similar technique for designating alternate column names, and in that situation the column alias name is actually called an alias in the standard.
Multitable Query Performance
As the number of tables in a query grows, the amount of effort required to carry out the query increases rapidly. The SQL itself places no limit on the number of tables joined by a query. Some low-end and embedded SQL products do limit the number of tables, with a limit of about eight tables being fairly common. The high processing cost of queries that join many tables imposes an even lower practical limit in many applications. In online transaction processing (OLTP) applications, it s common for a query to involve only one or two tables. In these applications, response time is critical the user typically enters one or two items of data and needs a response from the database within a second or two. Here are some typical OLTP queries for the sample database: The user enters a customer number into a form, and the DBMS retrieves the customer s credit limit, account balance, and other data (a single-table query). A cash register scans a product number from a package and retrieves the product s name and price from the database (a single-table query). The user enters a salesperson s name, and the program lists the current orders for that salesperson (a two-table inquiry). In decision-support applications, by contrast, it s common for a query to involve many different tables and to exercise complex relationships in the database. In these applications, the query results are often used to help make expensive decisions, so a query that requires several minutes or even many hours to complete is perfectly acceptable. Here are some typical decision-support queries for the sample database: The user enters an office name, and the program lists the 25 largest orders taken by salespeople in that office (a three-table query). A report summarizes sales by product type for each salesperson, showing which salespeople are selling which products (a three-table query). A manager considers opening a new Seattle sales office and runs a query analyzing the impact on orders, products, customers, and the salespeople who call on them (a four-table query). In the small tables of the sample database, even these queries would require only seconds to complete on low-cost computer hardware. But if the tables contained tens of millions of rows, the time to execute the queries would likely be much longer. The performance of multitable joins can be highly dependent on the index structures and other internal data structures that the DBMS uses to organize the data that it stores. In general, queries that exercise primary/foreign key relationships will perform fairly well, because the DBMS tends to optimize for those.
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