By Clare Churcher
This publication is for someone who has a well-designed relational database and wishes to extract
some details from it. you will have spotted within the prior sentence that the database
must be “well designed.” I can’t overemphasize this element. in case your database is badly
designed, it's going to no longer be ready to shop actual and constant facts, so the knowledge your
queries retrieve will continually be at risk of inaccuracies. while you're trying to layout a database
from scratch, you need to learn my first e-book, starting Database layout (Apress, 2007).
The ultimate bankruptcy of this ebook outlines a number of universal layout difficulties you're likely
to stumble upon and offers a few recommendation approximately easy methods to mitigate the influence or correct
For this booklet, you don't need any theoretical wisdom of relational idea, as I will
explain the proper concerns as they arrive up. the 1st bankruptcy supplies a short review of
relational database concept, however it may help when you've got had a few event operating with
databases with a couple of or extra tables.
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Extra info for Beginning SQL Queries. From Novice to Pro
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.
Beginning SQL Queries. From Novice to Pro by Clare Churcher