By Clare Churcher
An individual who does any paintings in any respect with databases must be aware of whatever of SQL. this can be a pleasant and easy-to-read advisor to writing queries with the all-important - within the database international - SQL language. the writer writes with unparalleled readability.
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Extra resources for Beginning SQL queries : from novice to professional
Listing 2-5. The Project Operation to Retrieve Names and Phone Numbers from the Member Table π 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.
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. 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.
We can see part of the Entry table in Figure 2-10. e Figure 2-10. The row variable e investigates each row independently. Imagine our finger is pointing at the row shown in the diagram. TourID= 38? It satisfies the second part, but the AND operator requires the row to satisfy both conditions. No single row in our table will have both 36 and 38 in the tournament column because each row is for just one entry. The SQL in Listing 2-29 will never find any rows; it will always return an empty table.