Beginning SQL

Chapter 4
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index helps speed up SELECT queries and WHERE clauses, why not always have one? First of all, while it
speeds up data retrieval, it slows down data input, with UPDATE and INSERT statements, for example.
Additionally, it adds to a database’s size, albeit not massively, though it is a consideration. Using indexes
is good practice, but it is best done judiciously — for example, using them to help your most common
queries.
Creating an index involves the CREATE INDEX statement, which allows you to name the index, to specify
the table and which column or columns to index, and to indicate whether the index is in ascending or
descending order. Indexes can also be unique, similar to the UNIQUE constraint, in that the index prevents
duplicate entries in the column or combination of columns on which there’s an index. The basic
format of the statement is as follows:
CREATE INDEX
ON ()
The following code adds an index called member_name_index on the MemberDetails table, and it
indexes the FirstName and LastName columns:
CREATE INDEX member_name_index
ON MemberDetails (FirstName, LastName);
If you execute the following SELECT statement, you notice something interesting:
SELECT FirstName, LastName
FROM MemberDetails;
The results, shown in the following table, appear in ascending order, by first name and then last name,
even though you did not add an ORDER BY clause:
FirstName LastName
Catherine Hawthorn
Doris Night
Jack Johnson
Jamie Hills
Jenny Jones
John Jackson
John Jones
Katie Smith
Seymour Botts
Steve Gee
Stuart Dales
Susie Simons
William Doors