How to put part of a SELECT statement into a Postgres function

问题

I have part of a SELECT statement that is a pretty lengthy set of conditional statements. I want to put it into a function so I can call it much more efficiently on any table I need to use it on.

So instead of:

SELECT 
    itemnumber, 
    itemname, 
    base, 
    CASE 
        WHEN labor < 100 AND overhead < .20 THEN
        WHEN .....
        WHEN .....
        WHEN .....
        .....
    END AS add_cost,
    gpm
FROM items1;

I can just do:

SELECT 
    itemnumber, 
    itemname, 
    base, 
    calc_add_cost(),
    gpm
FROM items1;

Is it possible to add part of a SELECT to a function so that can be injected just by calling the function?

I am sorting through documentation and Google, and it seems like it might be possible if creating the function in the plpgsql language as opposed to sql. However, what I am reading isn't very clear.

回答1:

You cannot just wrap any part of a SELECT statement into a function. But an expression like your CASE can easily be wrapped:

CREATE OR REPLACE FUNCTION  pg_temp.calc_add_cost(_labor integer, _overhead numeric)
  RETURNS numeric AS
$func$
SELECT 
    CASE 
        WHEN _labor < 100 AND _overhead < .20 THEN numeric '1'  -- example value
--      WHEN .....
--      WHEN .....
--      WHEN .....
        ELSE numeric '0'  -- example value
    END;
$func$
  LANGUAGE sql IMMUTABLE;

While you could also use PL/pgSQL for this, the demo is a simple SQL function. See:

Difference between language sql and language plpgsql in PostgreSQL functions

Adapt input and output data types to your need. Just guessing integer and numeric for lack of information.

Call:

SELECT calc_add_cost(1, 0.1);

In your statement:

SELECT 
    itemnumber, 
    itemname, 
    base, 
    calc_add_cost(labor, overhead) AS add_cost,  -- pass column values as input
    gpm
FROM items1;

You should understand some basics about Postgres functions to make proper use. Might start with the manual page on CREATE FUNCTION.

There are also many related questions & answers here on SO.

Also see the related, more sophisticated case passing whole rows here: