By Degen W., Boehmer K.

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I’ll use aliases in all the SQL queries from now on. fm Page 24 Thursday, March 6, 2008 3:00 PM 24 CHAPTER 2 ■ SIMPLE QUERIES ON ONE TABLE Combining Subsets of Rows and Columns In the previous sections, we saw the algebra operations select (a subset of rows) and project (a subset of columns) acting independently. One of the most powerful features of the algebra is that the result of an operation is another table (or, more formally, another set of unique rows). This means we can apply another operation to the result of the first operation and so build up complex queries.

Summary This chapter has presented an overview of relational databases. We have seen that a relational database consists of a set of tables that represent the different aspects of our data (for example, a table for members and a table for types). Each table has a primary key that is a field(s) that is guaranteed to have a different value for every row, and each field (or column) of the table has a set of allowed values (a domain). We have also seen that it is possible to set up relationships between tables with foreign keys.

Every row has a value for each attribute except for Handicap, which doesn’t apply to some members. Real data is usually not so clean. Let’s consider some different data, as in Figure 2-5. fm Page 30 Thursday, March 6, 2008 3:00 PM 30 CHAPTER 2 ■ SIMPLE QUERIES ON ONE TABLE Figure 2-5. Table with missing data When there is no value in a cell in a table, it is said to be Null. Nulls can cause a few headaches in a database. For example, if we ran two queries, one to produce a list of male members and the other a list of females, we might assume that all the members of the club would appear on one list or the other.

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Geloeste Aufgaben zur Diff. und Integralrechnung II by Degen W., Boehmer K.


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