create barcode image c# Figure 3-15. Result of a full outer join between Member and Type tables in Font

Drawing PDF-417 2d barcode in Font Figure 3-15. Result of a full outer join between Member and Type tables

Figure 3-15. Result of a full outer join between Member and Type tables
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CHAPTER 3 A FIRST LOOK AT JOIN S
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We have our row for Barbara Olson padded with Null values for the missing columns from the Type table. We also have the first row, which shows us the information about the associate membership type even though there are no rows in the Member table with Associate as a member type. Here each missing value from the Member table is replaced with a Null. Not all implementations of SQL necessarily have a full outer join implemented explicitly. Access 2007 doesn t. However, there are always alternative ways in SQL to retrieve the information you want. In 7 I ll show you how to get the equivalent of a full outer join by using a union operator between a left and right outer join (which is what I had to do to get the screen shot in Figure 3-15!).
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Summary
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Joins are the most often used of the operations that involve combining two tables. The resulting table has a column for each column in the two contributing tables. A join condition tells us which combinations of rows from the two contributing tables we will retain. The most common condition is equality between a field in each table. In the example in this chapter, the value of MemberType in the Member table had to equal the value of Type in the Type table. There are a number of equivalent ways of expressing joins in SQL. Table 3-1 shows a couple.
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Table 3-1. SQL for Simple Joins
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SELECT * FROM ON <table1> INNER JOIN <table2> <condition>
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SELECT * FROM <table1>, <table2> WHERE <condition>
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C HA P TER 3 A F IRS T LO O K A T JOI NS
If one (or both) of the tables has rows with a Null in the field involved in the join condition, then that row will not appear in the result of an inner join. If that row is required, you can use an outer join. A left outer join will include all rows in the left table, including those with a Null in the join field the corresponding fields from the right table will contain Nulls. A right outer join is the same but includes all rows from the right table, and a full outer join includes all rows from both tables.
Nested Queries
n the previous chapters, we looked at the select and project operations, which retrieve a subset of rows and columns from a single table, and we also looked at Cartesian products and joins, which combine two tables on a common field. We saw that there are several different ways to write SQL statements to perform combinations of these operations. As queries become more complicated, we might find that we can think of SQL expressions for different parts of the query but we need a way to tie them all together. In this chapter, we will look at nested queries and two new SQL keywords, EXISTS and IN. We will see how to use nesting to perform some of the queries we have already done and also how this will open up some other possibilities.
IN Keyword
The IN keyword allows us to select rows from a table, where an attribute can have one of several values. For example, if we wanted to retrieve the member IDs from the rows in our Entry table for tournaments with ID 36, 38, or 40, we could do this with a Boolean OR operator, as in the SQL statement in Listing 4-1.
Listing 4-1. Using OR Operations SELECT e.MemberID FROM Entry e WHERE e.TourID= 36 OR e.TourID= 38 OR e.TourID= 40
CHAPTER 4 NES TED QUER IES
Clearly, the sort of expression shown in Listing 4-1 will start to become unwieldy as the number of possible options grows. Using the IN keyword, we can construct a more compact statement, as in Listing 4-2, where the set of possible values are enclosed in parentheses and separated by commas. Each row of Entry is investigated, and if TourID is one of the values in the set, then the WHERE condition is true, and that row will be returned.
Listing 4-2. Using the IN Keyword SELECT e.MemberID FROM Entry e WHERE e.TourID IN (36, 38, 40)
We can combine IN with the logical operator NOT, as shown in Listing 4-3. The query will return all the IDs of members who have entered any tournament that is not in the list. We will look more carefully at using NOT later in the chapter.
Listing 4-3. Using NOT IN Keyword SELECT e.MemberID FROM Entry e WHERE e.TourID NOT IN (36, 38, 40)
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