WHERE Clause Rules

DataBee Extraction Set WHERE Clause rules are used to define the initial set of extracted rows for a table. In a sense, they provide the initial conditions for the extract. Think of it like this: DataBee uses the Table-To-Table rules to make sure every table contains the rows it needs in order to support the other tables. It is kind of similar to balancing a set of scales. If no tables have any rows then they will all be in balance - but the database is empty. The WHERE Clause rules provide the initial data to a few tables - this puts the system temporarily out of balance until the Table-To-Table rules are used. By the time the Table-To-Table rules have finished working, the system contains the appropriate rows in every table and a database subset has been extracted.

There can be any number of WHERE Clause rules providing any number of initial conditions.

Table-To-Table Rules

A DataBee extraction set Table-To-Table rule defines a data relationship between two tables. Essentially a Table-To-Table rule says " based on this join condition extract all the rows from table B needed to support the rows already retrieved for table A". In DataBee terminology table A would be known as the Parent table (its rows must be supported by another table), and table B would be the Child (its rows must support the Parent table). A shorthand notation for this rule is A->B. Note that the direction of the arrow always points from the Parent to the Child table and can be thought of as meaning "rows in Table A require rows to be in Table B".

In DataBee versions 3.0 (and greater) Table-To-Table rules can be defined between tables which are located in different schemas.

If table B also has a rule that require rows from another table (table C for example) the new rows just extracted into table B to satisfy the Table A to B rule (A->B) would be used as a base for the retrieval of the necessary rows from table C. It is quite possible (common really) for rule loops to occur (A->B, B->C then C->A). If such a loop occurs, and table C does have a rule that requires rows from table A, DataBee will retrieve the necessary additional rows from table A. The rows that table A already has will not be retrieved - doing so would set off an endless loop and be inefficient as well.

Of course because more rows have been retrieved for table A then table B will have to be re-checked (because of the A->B rule). DataBee will continue to cycle through its list of rules until all tables are in balance and no table has a rule that has not retrieved all of the rows it requires.