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OJB query

This document describes the use of different query mechanisms. The code in the documentation has passed JUnit testing.

Content:

1. Query by criteria

2. ODMG object Query Language(OQL)

3. JDO queries

Search by criteria:

In this section, you will learn how to query using criteria. The corresponding classes are in the org.apache.ojb.broker package

Medium. You can use criteria queries to get either the entire object (such as person) or report queries

to a single line of data.

A query consists of two main parts:

1. Get the object of the corresponding class

2. A list of conditions with ORDER BY and GROUP BY

OJB provides a QueryFactory class to create a new query. Although all query class constructors are public

However, we recommend using QueryFactory to create a new query:

Query q = QueryFactory.newQuery(Person.class, crit);

Each condition represents a sql-WHERE statement.

Criteria crit = new Criteria();

crit.addEqualTo("firstname", "tom");

crit.addEqualTo("lastname", "hanks");

Query q = QueryFactory.newQuery(Person.class, crit);

The corresponding SQL statement is as follows:

select ... FROM PERSON WHERE FIRSTNAME = "tom" AND LASTNAME = "hanks";

Query conditions:

OJB provides conditional selection of all SQL-comparators. In most cases, you don't have to do it yourself.

Classes like EqualToCriteria. The Criteria class provides corresponding methods, which are as follows:

1. Create criteria by comparing values of corresponding fields: e.g. addEqualTo ("firstname","tom")

2. Create criteria by comparing two fields: e.g. addEqualToField("firstname","other_fi

eld")

3. Create criteria by checking for nulls: e.g. addIsNull("firstname")

4. Create a fuzzy sql criteria: such as addSql ("REVERSE(name) like 're %'")

Here is how to compare the values of a field:

addEqualTo

addLike

addGreaterOrEqualThan

addGreaterThan

addLike

addbetween , this method requires two parameters

addIn , which uses the Collection class as a value argument

Here's how to compare two fields, both ending in... field:

addEqualToField

addGreaterThanField

and of course there negative forms

in/not in

Some databases limit the number of parameters in the IN syntax

If there is a limit, OJB will break the IN statement into several statements. The following example sets the limit number to 3:

SELECT ... FROM Artikel A0 WHERE A0.Kategorie_Nr IN ( ? , ? , ? )

OR A0.Kategorie_Nr IN ( ? , ? ) ORDER BY 7 DESC

The limits of IN can be defined in OJB.properties:

...

# The SqlInLimit entry limits the number of values in IN-sql

# statement, -1 for no limits. This hint is used in Criteria.

SqlInLimit=200

...

and/or

The query conditions obtained above are all "and" relations. Sometimes it is necessary to obtain "and" relations, as follows:

Criteria crit1 = new Criteria();

crit1.addLike("firstname", "%o%");

crit1.addLike("lastname", "%m%");

Criteria crit2 = new Criteria();

crit2.addEqualTo("firstname", "hank");

crit1.addOrCriteria(crit2);

Query q = QueryFactory.newQuery(Person.class, crit1);

Collection results = broker.getCollectionByQuery(q);

The corresponding SQL statement is as follows:

SELECT ... WHERE (FIRSTNAME LIKE "%o%") AND LASTNAME

LIKE "%m%" OR FIRSTNAME = "hank"

Sorting and grouping:

The following methods can be used to sort and group:

addOrderByAscending(String anAttributeName);

addOrderByDescending(String anAttributeName);

addGroupBy(String anAttributeName); This method is used to report queries

You can also do multiple sorting and grouping, repeat the call addOrderBy can:

crit = new Criteria();

crit.addOrderByDescending("id");

crit.addOrderByAscending("lastname");

query = new QueryByCriteria(Person.class, crit);

broker.getCollectionByQuery(query);

The above code queries all Persons in descending order of id and ascending order of lastname. This query will produce

Create the following SQL statement:

SELECT A0.ID,A0.FIRSTNAME,A0.LASTNAME FROM

PERSON A0 ORDER BY 1 DESC, 3

If you use LASTNAME instead of lastname, a new LASTNAME will be created automatically:

SELECT A0.ID,A0.FIRSTNAME,A0.LASTNAME,LASTNAME FROM PERSON A0 ORDER BY 1 DESC,

4

If more than one table contains LASTNAME, the SQL statement will report an error, so it is best to use the same as the attribute name.

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