如果查询未找到行，如何返回 null

Question

I am using SQL Server 2017. I am creating the whole MySQL Sakila database into mine in Azure Data Studio. I am currently stuck with functions, specifically with the line:

DECLARE EXIT HANDLER FOR NOT FOUND RETURN NULL;

This is my attempt, without the line that I mention it seems that the query has no errors.

CREATE FUNCTION inventory_held_by_customer (@p_inventory_id INT)
RETURNS int
WITH EXECUTE AS CALLER
AS
BEGIN
  DECLARE @v_customer_id INT;
  DECLARE EXIT HANDLER FOR NOT FOUND RETURN NULL; ---if the query finds no rows at all, then return a null
  SELECT @v_customer_id=customer_id
  FROM rental
  WHERE return_date IS NULL
  AND inventory_id = @p_inventory_id;

  RETURN(@v_customer_id);
END;

This is the original query for MySQL

CREATE DEFINER=`root`@`localhost` FUNCTION `inventory_held_by_customer`(p_inventory_id INT) RETURNS int(11)
READS SQL DATA
BEGIN
  DECLARE v_customer_id INT;
  DECLARE EXIT HANDLER FOR NOT FOUND RETURN NULL;

  SELECT customer_id INTO v_customer_id
  FROM rental
  WHERE return_date IS NULL
  AND inventory_id = p_inventory_id;

  RETURN v_customer_id;
END

Thanks in advance, I appreciate the help!

Answer 1

Firstly, you do not need to add DECLARE EXIT HANDLER FOR NOT FOUND RETURN NULL; to your function, it will return null if there is no result anyway.

Secondly, scalar functions are slow, because they are executed in a separate context on every single row. It is much more performant to create an inline TVF instead:

CREATE FUNCTION inventory_held_by_customer (@p_inventory_id INT)
RETURNS TABLE
AS RETURN
(
  SELECT customer_id
  FROM rental
  WHERE return_date IS NULL
  AND inventory_id = @p_inventory_id
);

GO