View source for How to define an on Create Default

==Objective==
The objective of this document is to briefly explain what an '''onCreateDefault''' is, in what situations it's useful, and to describe how to correctly define one.

==Concept==
OnCreateDefaults are SQL statements which are executed by the update.database task when a database column is created. They are normally used to insert data into a newly created column.
It's important to note that they are only executed when the column is being created. If the column is already in the database, update.database will not execute the onCreateDefault statement. To fill data into an already created column, you can use a Module Script. To find more about the Module Scripts, you can see [[How_to_create_build_validations_and_module_scripts| this]] document in our Developers Guide.

==Development process==
onCreateDefault statements are added directly to the XML file of the table. This means that the main development steps should be:
* Add the new column in the database
* Run export.database to export the column to the XML file
* Edit the xml file to add the onCreateDefault

An onCreateDefault statement should be valid SQL which can be appended in two different kinds of SQL commands. Let's assume that the table C_BPARTNER is going to be extended, adding a MYCOLUMN column. We need to design an onCreateDefault statement which works with the following two kinds of SQL commands:

<source lang="sql">
INSERT INTO C_BPARTNER (Column1, Column2, ..., MYCOLUMN) SELECT (Column1, Column2, ..., MYCOLUMN_ONCREATEDEFAULT) FROM C_BPARTNER_

UPDATE C_BPARTNER SET Column1=ValueForCol1, Column2=ValueForCol2, ..., MYCOLUMN=(MYCOLUMN_ONCREATEDEFAULT)
</source>

So, in this case, let's imagine that MYCOLUMN is a Yes/No column, and we want to set it as 'N' for the existing rows of C_BPARTNER when the column is first created. We would modify the column definition, so it looks like this:

<source lang="xml">
      <column name="MYCOLUMN" primaryKey="false" required="true" type="CHAR" size="1" autoIncrement="false">
        <default><![CDATA[N]]></default>
        <onCreateDefault><![CDATA['N']]></onCreateDefault>
      </column>
</source>

Notice the difference in syntax in the onCreateDefault, compared to the standard default. We need to add quotes there, because of the type of SQL command the onCreateDefault will be inserted in.
{{(!)| Importantant Notes:
* the onCreateDefault SQL statement '''must return only one row'''.

* the execution of the onCreateDefault takes place while the table is being rebuilt. This means that if the statement fails, there could be data loss in the customer database when the module is installed. Therefore '''the statement must be built so that it always can be executed'''.
}}
One limitation of the current implementation of onCreateDefaults is that, due to having to support the two different kinds of SQL syntax, you cannot make reference to a different column of the table you are modifying. This means that it can be very tricky to design a correct onCreateDefault in situations in which you need to make a link to a different table. For these situations, a Module Script could be used instead of an onCreateDefault, and the same result can be achieved this way.

==How to correctly remove an onCreateDefault==
onCreateDefaults are also removed by editing the XML file. The correct way to remove it is to delete the contents inside the <onCreateDefault> tag, but leave the tag itself there. If you delete everything, including the tag, next time you export the module (or Core, if you are editing a Core column), there will be an inconsistency, as [[DBSourceManager]] will create an empty onCreateDefault element.
In a practical example, the following column has an onCreateDefault:

<source lang="xml">
      <column name="MYCOLUMN" primaryKey="false" required="true" type="CHAR" size="1" autoIncrement="false">
        <default><![CDATA[N]]></default>
        <onCreateDefault><![CDATA['N']]></onCreateDefault>
      </column>
</source>

If we want to remove it, the correct way would be to leave the column like this:

<source lang="xml">
      <column name="MYCOLUMN" primaryKey="false" required="true" type="CHAR" size="1" autoIncrement="false">
        <default><![CDATA[N]]></default>
        <onCreateDefault/>
      </column>
</source>
[[Category:HowTo]]