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Listing 2-2. The Select Operation Used to Retrieve Members with Handicaps Under 12
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Handicap <12 (Member)
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Relational Calculus for Retrieving Rows
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The relational algebra describes how we should retrieve the required information from our database; in the previous cases, it s by saying Go and get the subset of rows that satisfy this condition. The relational calculus describes what the required information is like.
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CH A PT ER 2 S IMP LE QUER IES O N O NE T AB LE
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Listing 2-3 shows the shorthand for expressing the calculus for retrieving men from the Member table.
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Listing 2-3. The Calculus Expression to Retrieve All the Men from the Member Table {m | Member(m) and m.Gender = 'M'}
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The part on the right of the vertical bar (|) is the description of the retrieved rows. The expression in Listing 2-3 can be interpreted as saying I want a set of rows that come from the Member table, and each row must have M as the value for the Gender attribute. The letter m in Listing 2-3 is officially called a tuple variable, but I ll refer to it as a row variable (which is not strictly accurate but a bit easier to understand). I like to think of the variable acting like a finger, as in Figure 2-3. The finger (labeled m) points to each row in the Member table and checks to see whether it obeys the condition that its Gender attribute has the value M . As our queries get more complex, we will have many different fingers pointing at different tables.
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Figure 2-3. Row variable m investigating each row of the Member table
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SQL for Retrieving Rows
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Listing 2-4 shows the SQL statement for retrieving information about the men in our golf club.
Listing 2-4. The SQL Statement to Retrieve All the Men from the Member Table SELECT * FROM Member WHERE Gender = 'M'
This query has three parts, or clauses: The SELECT1 clause says what information to retrieve. In this case, * means retrieve all the columns. The FROM clause says which table(s)
1. Note that in SQL the keyword SELECT just means that a given statement is a query for retrieving information. It doesn t mean that the statement is necessarily going to involve an algebra select operation.
CHAPTER 2 SIMPLE QUERIES ON ONE TABLE
the query involves, and the WHERE clause describes the condition for deciding whether a particular row should be included in the result. Our condition says to check the value in the field Gender. In SQL when we specify an actual value for a character field, we need to enclose the value in single quotes, as in 'M'.
Retrieving a Subset of Columns
Now let s look at how we can specify that we want to see only some of the columns in our result, perhaps just names and phone numbers as in Figure 2-2b. Once again, this is an operation that we can apply to an original table in our database or to a virtual table resulting from some complex combination of several tables.
Relational Algebra for Retrieving Columns
The relational algebra operation for retrieving a subset of columns is project, and we represent it with the Greek letter pi ( ). Listing 2-5 shows the algebra for selecting the names and phone numbers from our Member table.
Listing 2-5. The Project Operation to Retrieve Names and Phone Numbers from the
Member Table
LastName, FirstName, Phone (Member)
The columns we want to retrieve (LastName, FirstName, and Phone) are specified in the subscript.
Relational Calculus for Retrieving Columns
Our notation for expressing a calculus query is in two parts separated by a bar, as in Listing 2-6. The part on the left describes what information we want to retrieve (in this case the LastName, FirstName, and Phone columns), and the part on the right describes the condition. In this case, the condition is only that the row comes from the Member table because we want the information for all our members.
Listing 2-6. The Calculus Expression to Retrieve Names and Phone Numbers from the
Member Table
{m.LastName, m.FirstName, m.Phone | Member(m) }
Once again, it is useful to think of the row variable m as being a finger pointing at each row, deciding whether it is to be included and then retrieving the specified attributes of that row.
CH A PT ER 2 S IMP LE QUER IES O N O NE T AB LE
SQL for Retrieving Columns
We specify what columns we want to retrieve in the SELECT clause of an SQL query. Whereas previously we used * to say return all the columns, Listing 2-7 now specifies the subset of columns we want in our result.
Listing 2-7. The SQL for Retrieving Names and Phone Numbers from the Member Table SELECT LastName, FirstName, Phone FROM Member
Because we want to see all these column values for every row, this query doesn t need a WHERE clause.
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