Codd’s Rules in Action: Simple SELECT Examples
Up to this point, the chapter has been about the individual aspects of a relational database platform as defined by Codd and implemented under ANSI SQL. This section presents a high-level overview of the most important SQL statement, SELECT , and some of its most salient points—namely, the relational operations known as projections, selections, and joins:
- Projection
Retrieves specific columns of data.
- Selection
Retrieves specific rows of data.
- Join
Returns columns and rows from two or more tables in a single result set.
Although at first glance it might appear that the SELECT statement deals only with the relational selection operation, in actuality, SELECT embodies all three operations. (Refer to SELECT Statement.)
The following statement embodies the projection operation by selecting the first and last names of an author, plus his home state, from the authors table:
SELECT au_fname, au_lname, state FROM authors
The results from any such SELECT statement are presented as another table of data:
au_fname au_lname state -------------------- ----------------------------------- ----- Johnson White CA Marjorie Green CA Cheryl Carson CA Michael O'Leary CA Meander Smith KS Morningstar Greene TN Reginald Blotchet-Halls OR Innes del Castillo MI
The resulting data is sometimes called a result set, work table, or a derived table, differentiating it from the base table in the database that is the target of the SELECT statement.
It is important to note that ...
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