Beginning SQL queries : from novice to professional by Clare Churcher

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By Clare Churcher

A person who does any paintings in any respect with databases must comprehend whatever of SQL. this can be a pleasant and easy-to-read consultant to writing queries with the all-important - within the database international - SQL language. the writer writes with extraordinary readability.

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LastName We can also order by two or more values. For example, if we want to order members with the same LastName by the value of their FirstName, we can include those two attributes (in that order) in the ORDER BY clause. The ORDER BY clause is the final clause in an SQL query. Listing 2-23 shows how to list all the senior members ordered by LastName and where the last names are the same by FirstName. Listing 2-23. FirstName The type of a field determines how the values will be ordered. Text fields will be ordered alphabetically, number fields will be ordered numerically, and date and time fields will be ordered chronologically.

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 (π).

NOT is used before an expression. For example, for our Member table, we might ask for rows obeying the condition NOT (MemberType = 'Social'). This means check each row, and if the value of MemberType is “Social”, then we don’t want that row. Table 2-3 gives some examples for the Member table. In the diagrams, each circle represents a set of rows (that is, those for social members or those for members with handicaps under 12). The shaded area represents the result of the operation. Table 2-3. fm Page 29 Thursday, March 6, 2008 3:00 PM CH A PT ER 2 ■ S IMP L E QUER IES O N O NE T AB LE The little truth tables in Figure 2-4 can be helpful in understanding and remembering how the Boolean operators work.

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