Add function parameter to SQL query WHERE clause

I have a web application in java and it uses a query. I don't want to write the query into Java, so I made a function:

CREATE OR REPLACE FUNCTION testFunc(inputs text) RETURNS TABLE(...) AS 
$
    SELECT .... FROM ...
    JOIN ...
    where true
    ;
$
LANGUAGE SQL;

I want the Function parameter INPUTS to also be in the WHERE clause so if inputs is

AND speed = 0 AND ....

Where clause looks like

where true AND speed = 0 AND ...

How can i achieve this?

EDIT

Also it is acceptable to have many parameters (a int, b string, c string ..) but then i need to have

WHERE speed = * AND stop = * AND ...

which is not acceptable. How can i achieve this ?

OR can i put a if statement inside it ? Like

Select .. . from ...
JOIN ... 
WHERE true
IF (a != null){AND speed = $1}
IF ....
;

If you don't always pass all parameters, create a function with parameter defaults. The basic, simple form would be an SQL function without dynamic SQL:

CREATE OR REPLACE FUNCTION func(_a int  = NULL
                              , _b text = NULL
                              , _c text = NULL)
  RETURNS TABLE(...) AS
$func$
    SELECT ... FROM ...
    WHERE (speed = $1 OR $1 IS NULL)
    AND   (stop  = $2 OR $2 IS NULL)
    ...
$func$  LANGUAGE sql;

Now you can call the function with any number of parameters using named notation:

SELECT * FROM func(_c => 'foo', _a => 123);  -- no _b, which defaults to NULL

Note, the assignment operator in the call is => (or := for Postgres 9.4 or older), not =!

See:

• The forgotten assignment operator “=” and the commonplace “:=”

• Functions with variable number of input parameters

Aside: "string" is not a data type, text is.

Much more is possible with dynamic SQL with EXECUTE in a plpgsql function.

More comprehensive answer on SO with complete recipes:

• Test for is null or = value in function

StackExchange Database Administrators Q#85970, answer score: 9