Adding Variables to Selection Conditions (GETVAR and SETVAR)¶

There are situations where we want to create an aggregation view with a condition in it. That is, creating a view with a WHERE condition and a GROUP BY. The limitation of this is that the WHERE condition is static and cannot be changed at runtime.

For example, if we have two views:

1. A base view CLIENT with these fields: name, income and state.

2. And a view WEALTHY_CLIENT_BY_STATE defined as:

   CREATE VIEW WEALTHY_CLIENT_BY_STATE AS
   SELECT state, COUNT(*)
   FROM client
   WHERE income > 1000000
   GROUP BY state
   

There is a limitation in the second view: the limit of income to consider a client wealthy is static. So, we have to know this limit before creating the view. If we wanted to change this limit at runtime, we could remove the WHERE condition and add the field income to the GROUP BY fields. But then, we would be grouping by this field and we might not want to do that. Besides, if income is not in the output of the base view you cannot add income to the GROUP BY.

To avoid this problem, you can use the function GETVAR in the definition of the query. The syntax of this function is

GETVAR('<name of the variable>', '<type of the variable>', '<default value>')

GETVAR tries to obtain the value of the variable <name of the variable> from the CONTEXT of the query. If it does not find it, it returns <default value>.

For example, you could define the view WEALTHY_CLIENT_BY_STATE like this:

CREATE VIEW WEALTHY_CLIENT_BY_STATE AS
SELECT state, COUNT(*)
FROM client
WHERE income >= GETVAR('_var_wealthy_client_income_limit', 'int',
1000000)
GROUP BY state

With this change, the limit of income is no longer static and we can query the view defining this value at runtime:

SELECT * FROM WEALTHY_CLIENT_BY_STATE
CONTEXT ('VAR _var_wealthy_client_income_limit' = '250000')

If we do not put a value for the variable in the CONTEXT of the query, the value used in the selection condition is the <default value> of the GETVAR function: 1000000.

Another option is obtaining the value of a variable from another view at runtime and putting this value in the CONTEXT with the function SETVAR. The syntax of this function is:

SETVAR('<name of the variable>', '<value of the variable')

E.g. we have a DF base view INCOME_LIMIT that returns one row with the value that we want to use for the variable _var_wealthy_client_income_limit.

SELECT WEALTHY_CLIENT_BY_STATE.*
FROM
    (SELECT SETVAR('_var_wealthy_client_income_limit', limit)
    FROM INCOME_LIMIT WHERE type = 'wealthy')
NESTED ORDERED JOIN
    WEALTHY_CLIENT_BY_STATE;

We execute a NESTED JOIN between the two views because in this type of join, the left branch is executed first. That means that the Server queries the view INCOME_LIMIT first and the function SETVAR puts the value of the variable in the CONTEXT. Then, when the right branch is executed, GETVAR will find the value of the variable _var_wealthy_client_income_limit in the CONTEXT.