Download PDF by Clare Churcher: Beginning SQL queries : from novice to professional

By Clare Churcher

ISBN-10: 1430205504

ISBN-13: 9781430205500

ISBN-10: 1590599438

ISBN-13: 9781590599433

A person who does any paintings in any respect with databases must understand whatever of SQL. it is a pleasant and easy-to-read consultant to writing queries with the all-important - within the database international - SQL language. the writer writes with unprecedented readability.

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Extra info for Beginning SQL queries : from novice to professional

Sample text

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. fm Page 22 Thursday, March 6, 2008 3:00 PM 22 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.

Let’s look at an example. Consider the relational project operation to retrieve just the FirstName column from the Member table. Figure 2-7 shows two possible results. fm Page 33 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 a) With duplicates b) Without duplicates Figure 2-7. Projecting the FirstName column from the Member table From a mathematical perspective, there is no question that in terms of the relational algebra the project operation will give us Figure 2-7b, a set of unique rows with the duplicates for William and Thomas removed.

Let’s start with the Cartesian product: we want a set of rows made up of combinations of rows from each of the contributing tables. Figure 3-4 shows how we can envisage this. We are looking at two tables, so we need two fingers to keep track of the rows. Finger m looks at each row of the Member table in turn. Currently it is pointing at row 3. For each row in the Member table, finger t will point to each row in the Type table. t m Member table Type table Figure 3-4. Row variables m and t point to each row of their respective tables.

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Beginning SQL queries : from novice to professional by Clare Churcher


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