vb.net qr code sample Try It Out: Using a Searched CASE Expression in VB.NET

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Try It Out: Using a Searched CASE Expression
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Let s modify the simple CASE example to demonstrate searched CASE. Enter the following query into SSMSE and execute it. You should see the results in Figure 11-11.
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select distinct year(orderdate) NumYear, case when year(orderdate) = ( select max(year(orderdate)) from orders ) then 'Last year'
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CHAPTER 11 s LEARNING MORE ABOUT QUERIES
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else 'Prior year' end LabYear from orders
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Figure 11-11. Using a searched CASE expression
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The original query, though it worked, was severely limited in that it works correctly only if 1998 is really the last year for orders. You corrected this flaw with a searched CASE. Now the query will do the right thing, whatever years are in the Orders table! You replaced the numeric literal <when operand>, 1998, with a predicate (which can be just as complex as any predicate in a WHERE clause):
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year(orderdate) = ( select max(year(orderdate))
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CHAPTER 11 s LEARNING MORE ABOUT QUERIES
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from orders )
This predicate included a subquery. Remember, subqueries are simply queries embedded in other queries. Here, one is embedded in a CASE expression rather than in an IN predicate (as demonstrated earlier in the chapter). The value returned by the subquery is the maximum year in the Orders table, so whenever you run the query, you ll get the correct last year without ever having to know what it is.
s Note Complex queries are a normal part of database applications. The more you learn about SQL, the
better you ll be able to exploit its considerable power. All major DBMSs have query optimizers that can find efficient access paths for even complex queries. You should code whatever complexity you need, relying on the optimizer to do its job; however, even simple queries can sometimes be inefficient, depending on how they re coded. In addition to learning SQL, learn whatever tool your DBMS offers to analyze query access paths.
You ve merely scratched the surface of the many, many facilities SQL offers for coding complex, highly sophisticated queries. Let s now look at the most important one.
Joins
Most queries require information from more than one table. A join is a relational operation that produces a table by retrieving data from two (not necessarily distinct) tables and matching their rows according to a join specification. Different types of joins exist, which you ll look at individually, but keep in mind that every join is a binary operation; that is, one table is joined to another, which might be the same table because tables can be joined to themselves. The join operation is a rich and somewhat complex topic. The next sections will cover the basics.
Inner Joins
An inner join is the most frequently used join. It returns only rows that satisfy the join specification. Although in theory any relational operator (such as > or <) can be used in the join specification, almost always the equality operator (=) is used. Joins using the equality operator are called natural joins.
CHAPTER 11 s LEARNING MORE ABOUT QUERIES
The basic syntax for an inner join is as follows:
SELECT <select list> FROM left-table INNER JOIN right-table ON <join specification>
Notice that INNER JOIN is a binary operation, so it has two operands, left-table and right-table, which might be base tables or anything that can be queried (for example, a table produced by a subquery or by another join). The ON keyword begins the join specification, which can contain anything that could be used in a WHERE clause.
Try It Out: Writing an Inner Join
Let s retrieve a list of orders, the IDs of the customers who placed them, and the last name of the employees who took them. Enter the following query into SSMSE and execute it. You should see the results in Figure 11-12.
select orders.orderid, orders.customerid, employees.lastname from orders inner join employees on orders.employeeid = employees.employeeid
CHAPTER 11 s LEARNING MORE ABOUT QUERIES
Figure 11-12. Using INNER JOIN
How It Works
Let s start with the select list.
select orders.orderid, orders.customerid, employees.lastname
Because you re selecting columns from two tables, you need to identify which table a column comes from, which you do by prefixing the table name and a dot (.) to the column name. This is known as disambiguation, or removing ambiguity so the database manager knows which column to use. Though this has to be done only for columns that appear in both tables, the best practice is to qualify all columns with their table names. The following FROM clause specifies both the tables you re joining and the kind of join you re using:
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