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Subqueries in Java Integrated UPCA in Java Subqueries




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Subqueries use applet upc-a supplement 5 encoder tocompose upc a for java ANSI/AIM Code 93 A select state GTIN - 12 for Java ment that is embedded within another select statement is called a subquery. In the following sections I discuss several variations on this idea. Most of the time you do not have to use subqueries.

There are still a few places where you have to use them, and you may have to understand code that was written 10 years ago when outer joins were not available and inner joins were less efficient than they are today.. SUBQUERIES 18-12 Introduction to subqueries When SQL was f UPC A for Java irst created, people thought that subqueries would be the most important feature of the language. One style of coding SQL, found in older code, makes extensive use of subqueries. Some people still write code this way, but this style has now fallen out of favor.

It has largely been replaced by a style that prefers to use joins when there is a choice. There are three reasons for this change. One reason is that the processing of joins has become much more efficient.

Originally it was thought that subqueries would always process much more quickly than joins. In the past few years many improvements have been made to the Optimizer that make joins more efficient. Now joins are often just as efficient as subqueries, and sometimes even more efficient.

The second reason is that outer joins have now become a standard part of the language. The early SQL standards, SQL-86 and SQL-89, did not include outer joins. Subqueries were used to write them.

Now most products support outer joins and they are included in the newest standard, SQL-92. Now we can write an outer join without using a subquery. The third reason is that code written with many subqueries is difficult to understand and maintain.

Code written with joins is often easier to understand and modify when changes are needed, and other people can work with the code more easily.. Task Show an exampl jar GTIN - 12 e of a subquery. List the foods that cost less than the average price of all the items on the menu. List the descriptions and prices of these foods.

Sort the rows in ascending order on the description column.. Oracle & Access SQL select a.descr awt upc a iption, a.price from l_foods a where a.

price < (select avg(b.price) from l_foods b) order by a.description;.

CHAPTER 18 IF-THEN-ELSE, PARAMETER QUERIES, AND SUBQUERIES Beginning tabl UPC Code for Java e (l_foods table). SUPPLIER ID -- -----ASP ASP ASP CBC CBC FRV FRV JBR JBR VSB PRODUCT MENU PRICE CODE ITEM DESCRIPTION PRICE INCREASE ------- ------- -------------------- ----------- -------FS 1 FRESH SALAD $2.00 $0.25 SP 2 SOUP OF THE DAY $1.

50 (null) SW 3 SANDWICH $3.50 $0.40 GS 4 GRILLED STEAK $6.

00 $0.70 SW 5 HAMBURGER $2.50 $0.

30 BR 6 BROCCOLI $1.00 $0.05 FF 7 FRENCH FRIES $1.

50 (null) AS 8 SODA $1.25 $0.25 VR 9 COFFEE $0.

85 $0.15 AS 10 DESSERT $3.00 $0.

50. How the computer processes this query Step 1: The subquery is processed first. select avg(b.p awt upc barcodes rice) from l_foods b;. Result table: Step 1 AVG(B.PRICE) -----------2.31.

Step 2: The result is placed in the outer query. select a.descr javabean UPC-A Supplement 5 iption, a.price from l_foods a where a.

price < 2.31 order by a.description;.

Step 3: The outer query is processed. Result table DESCRIPTION PR applet UPCA ICE -------------------- ----------BROCCOLI $1.00 COFFEE $0.85 FRENCH FRIES $1.

50 FRESH SALAD $2.00 SODA $1.25 SOUP OF THE DAY $1.

50. SUBQUERIES Check your understanding Write an SQL q uery using a subquery. List all the employees who have a credit limit that is greater than the average credit limit..

18-13 Subqueries that result in a list of values Some subquerie s result in a single value and others return a list of values. Those are the only options. A subquery is not allowed to return any more values than that.

For example, it cannot return a table with several rows and several columns. The reason is that the result of the subquery must fit into the statement of the outer query. Within the context of an SQL statement, only a single value or a list of values can make sense.

This section shows a subquery that results in a list of values. To make this list work with the syntax of the outer join, the condition in the where clause must be: or.
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