OQL From Clause

Last update: Download PDF Edit

1 Introduction

The FROM clause specifies the entities or other source from which the data must be retrieved. This clause starts with the FROM keyword, following by an entity name. To select data from other entities as well, add these entities via the JOIN keywords. This syntax is a little more strict than the official SQL FROM clause syntax.

This is an example of the full syntax:

FROM
    {
        entity_name | ( sub_oql_query )
    }
    [ [ AS ] from_alias ]

    {
        { INNER | { { LEFT | RIGHT | FULL } [ OUTER ] } } JOIN
        entity_path [ [ AS ] from_alias ]
        ON <constraint>
    } [ ,...n ]

2 entity_name

This specifies the entity from which data must be retrieved.

3 ( sub_oql_query )

This specifies another OQL query from which data must be retrieved. This will be the source for the current query. The subquery must be placed within parentheses.

4 JOIN

This specifies a path to entity to join into this query. There are four different types of JOINS supported:

  • INNER JOIN
  • LEFT OUTER JOIN
  • RIGHT OUTER JOIN
  • FULL JOIN

The syntax is as follows:

 { INNER | { { LEFT | RIGHT | FULL } [ OUTER ] } } JOIN
        entity_path [ [ AS ] from_alias ]
        [ ON <constraint> ]

4.1 entity_path

This specifies the entity to join and the path from an earlier defined entity in the FROM clause to this entity.

The path Crm.Customer/Crm.Customer_Address/Crm.Address defines a path from the earlier defined entity Crm.Customer to the new entity Crm.Address.

4.2 [ ON <constraint> ]

This constrains the specified entity in the JOIN part of the FROM clause. The constraint syntax is similar to that of the WHERE clause. Only the entities and from aliases from the current and preceding JOIN elements can be used in the constraint.

This part is optional. The system will generate the appropriate JOIN condition based on the specified entity_path.

4.3 JOIN Types

4.3.1 INNER JOIN

An INNER JOIN is the most common join operation between entities and represents the default join type. The query compares each row of entity A with each row of entity B to find all the pairs of rows that have an association and satisfy the JOIN predicate. If the association exists and the JOIN predicate is satisfied, the column values for each matched pair of rows of A and B are combined into a resulting row.

The syntax is as follows:

[ INNER ] JOIN entity_path [ ON <constraint> ]

4.3.2 LEFT OUTER JOIN

With a LEFT OUTER JOIN construction, the query compares each row of entity A with each row of entity B to find all pairs of rows which have an association and thus satisfy the JOIN predicate. When the association exists and the the JOIN predicate is satisfied, column values for each matched pair of rows of A and B are combined into a resulting row.

However, in contrast to the INNER JOIN construction, the query will also return rows of entity A which do not match entity B. When columns of entity B are specified, these columns contain a null value for these rows.

The syntax is as follows:

LEFT [ OUTER ] JOIN entity_path [ ON <constraint> ]

4.3.3 RIGHT OUTER JOIN

With a RIGHT OUTER JOIN construction, the query compares each row of entity A with each row of entity B to find all pairs of rows which have an association and thus satisfy the JOIN predicate. If the association exists and the JOIN predicate is satisfied, the column values for each matched pair of rows of A and B are combined into a resulting row.

However, in contrast to the INNER JOIN construction, rows from entity B that do not match entity A will also be returned. When columns of entity A are specified, these columns contain a null value for these rows.

The syntax is as follows:

RIGHT [ OUTER ] JOIN entity_path [ ON <constraint> ]

4.3.4 FULL OUTER JOIN

With a FULL OUTER JOIN construction, the query compares each row of entity A with each row of entity B to find all pairs of rows which have an association and thus satisfy the join-predicate. When the association exists and the join-predicate is satisfied, column values for each matched pair of rows from A and B are combined into a result row.

However, in contrast to the INNER JOIN construction, data from entities that do not match will also be returned. For these rows, columns of missing entities will contain null values.

The syntax is as follows:

FULL [ OUTER ] JOIN entity_path [ ON <constraint> ]

4.4 Example

In this scenario, you are using a LEFT OUTER JOIN to get the records in table A that have no association in table B.

For example, you have the entities Customer and Order, where a customer can have an association to multiple orders. You want to retrieve all the customers that have no orders at all.

SELECT 
  Customer/Name as Name,
  Customer/<anyotherattribute> as <anyotherattribute>
FROM MyModule.Customer
  LEFT OUTER JOIN Customer/MyModule.Customer_Order/MyModule.Order as Order
WHERE Order/ID IS NULL