Simple Example; A More Complex Example; The Most Complex Example - HP NonStop SQL/MP Programming Manual

Data Retrieval and Modification

Simple Example

In this example, the search is performed on one column, which is the primary key of
the table. For example, a cursor SELECT to retrieve all the columns in the EMPLOYEE
table by primary key.
The WHERE clause in this example selects on a primary key value. This means that
the SQL compiler can choose the primary index as the access path so that each
FETCH statement returns the next row in primary key sequence. This code is simple
and efficient:
SELECT EMPNUM, FIRST_NAME, LAST_NAME, DEPTNUM, SALARY
FROM =EMPLOYEE
WHERE EMPNUM > :LASTEMPNUM
ORDER BY EMPNUM

A More Complex Example

In a slightly more complex example, suppose that the search uses a column that is not
the primary key (for example, the column LAST_NAME). In this case, the query should
be faster if there is an index on LAST_NAME. Suppose that there is an index on
LAST_NAME in this example:
SELECT EMPNUM, FIRST_NAME, LAST_NAME, DEPTNUM, SALARY
FROM =EMPLOYEE
WHERE LAST_NAME> :LAST-LNAME
ORDER BY LAST_NAME
When an index on a nonkey column is efficient and available, the SQL compiler
probably chooses that index.

The Most Complex Example

A more complex problem occurs when the key is composed of multiple columns. In this
case, you should generally use a multivalue predicate for the comparison. This type of
predicate compares multiple columns with multiple values.
Suppose that you want to retrieve the next row in sequence by last name and first
name, and an index exists on the two columns containing the last name and the first
name. Code this type of request by using a multivalue predicate. A multivalue
predicate allows you to concatenate two or more columns and compare them with two
or more concatenated values. This type of predicate retrieves the next name in
sequence. For example:
SELECT EMPNUM, FIRST_NAME, LAST_NAME, DEPTNUM, SALARY
FROM =EMPLOYEE
WHERE (LAST_NAME, FIRST_NAME) > :LAST_NAME, :FIRST_NAME
ORDER BY LAST_NAME, FIRST_NAME
If there is an index on the two columns LAST_NAME and FIRST_NAME in that order,
this query is probably as efficient as it can be.
HP NonStop SQL/MP Programming Manual for C—429847-008
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Simple Example